Scalable continuous flow microbial fuel cells

ABSTRACT

Disclosed are modular microbial fuel cell (MFC) devices, systems and methods for treating wastewater and generating electrical energy through a bioelectrochemical waste-to-energy conversion process. In some aspects, a modular MFC system includes a wastewater pretreatment system to receive and pre-treat raw wastewater for feeding pre-treated wastewater for bioelectrochemical processing; one or more modular MFC devices to bioelectrochemically process the pre-treated wastewater by concurrently generating electrical energy and digesting organic contaminants and particulates in the wastewater to yield treated, cleaner water; and a water collection module to receive the treated water from the one or more modular MFC devices and store the treated water and/or route the treated water from the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/457,455, entitled “SCALABLE CONTINUOUS FLOW MICROBIAL FUEL CELLS”filed Jun. 28, 2019, which claims priorities to and benefits of U.S.Provisional Patent Application No. 62/691,426 entitled “SCALABLECONTINUOUS FLOW MICROBIAL FUEL CELL SYSTEMS, DEVICES AND METHODS” filedon Jun. 28, 2018, and of U.S. Provisional Patent Application No.62/745,896 entitled “SCALABLE CONTINUOUS FLOW MICROBIAL FUEL CELLS”filed on Oct. 15, 2018. The entire content of the aforementioned patentapplications is incorporated by reference as part of the disclosure ofthis patent document.

TECHNICAL FIELD

This patent document relates to microbial fuel cell technology.

BACKGROUND

A microbial fuel cell (MFC) is a bioelectrochemical system that usesliving cells, such as bacteria, and mimics and/or promotes their naturalinteractions to produce electric current. Some example MFC systemsinclude mediated MFCs, which use a mediator for transferring electronsfrom the bacteria cell to the anode. Other MFC systems includeunmediated MFCs, which utilize types of bacteria that typically haveelectrochemically active redox proteins (e.g., cytochromes) on theirouter membrane that can transfer electrons directly to the anode.

SUMMARY

Disclosed are modular microbial fuel cell (MFC) devices, systems andmethods for treating wastewater and generating electrical energy througha bioelectrochemical waste-to-energy conversion process.

In some aspects, a system for energy generation and wastewater treatmentincludes a wastewater headworks system to pre-treat raw wastewater byremoving solid particles and produce a pre-treated wastewater that thatis outputted from the wastewater headworks system; one or more modularmicrobial fuel cell (MFC) devices to bioelectrochemically process thepre-treated wastewater by concurrently generating electrical energy anddigesting organic contaminants and particulates in the pre-treatedwastewater to yield a treated water, the one or more modular MFC devicescomprising a housing and a bioelectrochemical reactor that is encasedwithin the housing, wherein the bioelectrochemical reactor includes aplurality of anodes arranged between a cathode assembly; and a watercollection system to receive the treated water from the one or moremodular MFC devices and store the treated water and/or route the treatedwater from the system.

In some aspects, a method for energy generation and wastewater treatmentincludes pretreating a raw wastewater by removing at least some solidparticles from a wastewater fluid that produces a pre-treatedwastewater; processing the pre-treated wastewater by abioelectrochemical conversion process that generates electrical energyand concurrently cleans the pre-treated wastewater to produce treatedwater by digesting matter in the wastewater fluid; extracting thegenerated electrical energy for storage or transfer to an externalelectrical device; and outputting the treated water.

In some aspects, a device for energy generation and wastewater treatmentincludes a modular microbial fuel cell (MFC) device operable tobioelectrochemically process wastewater that includes organic matter ina fluid that concurrently generates electrical energy and digests theorganic matter to yield a treated water, the modular MFC devicecomprises: a housing, and a bioelectrochemical reactor encased withinthe housing, the bioelectrochemical reactor including a plurality ofanodes arranged between a cathode assembly, wherein the cathode assemblyincludes two gas-diffusion cathodes separated on two sides of theplurality of anodes and arranged longitudinally along a flow directionof the fluid through the bioelectrochemical reactor, the gas-diffusioncathodes able to allow oxygen to permeate into the fluid within thebioelectrochemical reactor.

In some aspects, a device for energy generation and wastewater treatmentincludes a first modular microbial fuel cell (MFC) device and a secondmodular MFC device. The first modular MFC device is operable tobioelectrochemically process wastewater that includes organic matter ina fluid that concurrently generates electrical energy and digests theorganic matter to produce a treated water, and the first modular MFCdevice comprises: a first housing, and a first bioelectrochemicalreactor encased within the first housing, the first bioelectrochemicalreactor including a plurality of anodes arranged between a cathodeassembly, wherein the cathode assembly includes two gas-diffusioncathodes separated on two sides of the plurality of anodes and arrangedlongitudinally along a flow direction of the fluid through the firstbioelectrochemical reactor, the gas-diffusion cathodes able to allowoxygen to permeate into the fluid within the first bioelectrochemicalreactor. The second modular MFC device is fluidically coupled to thefirst modular MFC device and operable to bioelectrochemically processthe treated water produced by the first modular MFC device toconcurrently generate electrical energy and digest organic matter influid of the treated water to produce a further treated water, and thesecond modular MFC device comprises: a second housing, and a secondbioelectrochemical reactor encased within the second housing, the secondbioelectrochemical reactor including a plurality of anodes arrangedbetween a cathode assembly, wherein the cathode assembly includes twogas-diffusion cathodes separated on two sides of the plurality of anodesand arranged longitudinally along a flow direction of the fluid throughthe second bioelectrochemical reactor, the gas-diffusion cathodes ableto allow oxygen to permeate into the fluid within the secondbioelectrochemical reactor.

The subject matter described in this patent document can be implementedto provide one or more of the following features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of an example embodiment of a modular microbialfuel cell (MFC) system in accordance with the present technology fornet-zero energy consuming wastewater treatment.

FIGS. 2A and 2B show diagrams of an example embodiment of an MFC deviceof the array of MFC devices shown in FIG. 1.

FIG. 2C shows an illustrative diagram of a bioelectrochemicalwaste-to-energy conversion process in accordance with the presenttechnology.

FIG. 3 shows a diagram of an example embodiment of the modular MFCsystem shown in FIG. 1 implemented for continuous treatment of swinewaste at high treatment rates.

FIG. 4 shows a diagram of an example embodiment of a modular MFC systemshown in FIG. 1 implemented for continuous treatment of domesticwastewater at high treatment rates.

FIGS. 5A-5E show diagrams for various views of an example embodiment asingle bioelectrochemical reactor in accordance with the presenttechnology.

FIG. 6A shows a diagram depicting various views of an example embodimentof a single bioelectrochemical reactor in accordance with the presenttechnology.

FIG. 6B shows examples of various views of an anode and a cathode of thesingle bioelectrochemical reactor of FIG. 6A.

FIG. 6C shows examples of fitment of an anode assembly into a housing ofthe single bioelectrochemical reactor of FIG. 6A.

FIG. 6D shows additional examples of fitment of an anode assembly into ahousing of the single bioelectrochemical reactor of FIG. 6A.

FIG. 6E shows examples of a housing for the single bioelectrochemicalreactor of FIG. 6A with anodes in place.

FIGS. 7A-7E show diagrams for various views of an example embodiment adouble bioelectrochemical reactor in accordance with the presenttechnology.

FIGS. 8A-8E show diagrams for various views of an example embodiment atriple bioelectrochemical reactor in accordance with the presenttechnology.

FIGS. 9A and 9B show data plots depicting current density for eachindividual reactor of the example MFC system of FIG. 3 when normalizedto the cathodes surface area (FIG. 9A) and the reactor volume (FIG. 9B).

FIG. 10 shows a data plot depicting open circuit potential (OCP)measurements of the anode and the cathodes for reactor five of theexample MFC system of FIG. 3.

FIGS. 11A and 11B show data plots depicting polarization and powercurves (FIG. 11A) and polarization curves for the cathode and anode ofreactor five during batch mode of operation (FIG. 11B).

FIGS. 12A-12D show data plots depicting polarization (FIGS. 12A, 12C)and power curves (FIG. 12B) as well as maximum power development overtime (FIG. 12D) for reactor 5 during continuous mode.

FIG. 13 shows a data plot depicting an EIS of a reactor of the exampleMFC system of FIG. 3 at different time intervals.

FIGS. 14A and 14B show data plots depicting COD concentration of theinfluent, effluent and the COD removal for the experimentalimplementation (FIG. 14A) and COD treatment rate (FIG. 14B) over time.

FIG. 15 shows a data plot depicting COD removal efficiency (%) duringcontinuous mode of system operation.

FIGS. 16A-16C show data plots depicting concentration of NO₃ ⁻—N (FIG.16A), NO₂ ⁻—N (FIG. 16B) and NH₄ ⁺—N (FIG. 16C) in the influent (in) andeffluent (out) of the example MFC system of FIG. 3 during continuousmode.

FIG. 17 shows a data plot depicting sulfate concentration in theinfluent (in) and effluent (out) of the example MFC system of FIG. 3during continuous mode.

FIG. 18 shows a data plot depicting pH and DO profiles over time.

FIG. 19 shows a plot depicting a microbial community profile of theexample MFC system effluent using 16S rRNA sequences.

FIG. 20 shows a plot depicting a 16S rRNA-based microbial communityprofile of samples collected from the anode and cathodes of reactor sixof the example MFC system of FIG. 3 after its decommissioning.

FIGS. 21A and 21B show SEM images showing biofilm on the anode (FIG.21A) and the cathode (FIG. 21B) of reactor six of the example MFC systemof FIG. 3.

FIGS. 22A and 22B show data plots including a CCA biplot (FIG. 22A) ofsamples collected from the system effluent, and a PCA biplot (FIG. 22B)of samples collected during the decommissioning of reactor six of theexample MFC system of FIG. 3.

FIG. 23 shows images of an example implementation of the example modularMFC system of FIG. 4 for continuous treatment of domestic wastewater athigh treatment rates.

FIG. 24A shows a data plot depicting current density for each individualreactor of the top treatment train of the example MFC system of FIG. 4when normalized to the cathodes surface area.

FIG. 24B shows a data plot depicting current density for each individualreactor of the bottom treatment train of the example MFC system of FIG.4 when normalized to the cathodes surface area.

FIG. 25 shows a data plot depicting an open circuit potential data forindividual reactor box 7B over time.

FIG. 26 shows a data plot depicting COD removal efficiency for top andbottom treatment train of the example MFC system of FIG. 4.

FIG. 27 shows a data plot depicting changes in nitrite and ammoniumconcentrations over time for top treatment train.

FIG. 28 shows a data plot depicting changes in sulfide and sulfateconcentrations over time for top treatment train.

FIG. 29 shows a data plot depicting pH trend over time of the top andbottom treatment train.

FIG. 30 shows a diagram of an example embodiment of a modular MFC systemin accordance with the present technology implemented for treatment ofbrewery wastewater.

FIG. 31 shows a data plot depicting current density for each individualreactor of the example MFC system of FIG. 30 when normalized to thecathodes surface area.

FIG. 32 shows a data plot depicting the open circuit potential data forindividual reactor box 11 over time.

FIG. 33A shows a data plot depicting COD removal rate as mg/L COD forthe example MFC system shown in FIG. 30 under batch mode.

FIG. 33B shows a data plot depicting COD removal rate efficiency for theexample MFC system shown in FIG. 30 under batch mode.

FIG. 34A shows a data plot depicting COD removal rate as mg/L COD forthe example MFC system shown in FIG. 30 under continuous mode.

FIG. 34B shows a data plot depicting COD removal rate efficiency for theexample MFC system shown in FIG. 30 under continuous mode.

Like reference numerals refer to the same or similar features.

DETAILED DESCRIPTION

Microbial fuel cells (MFCs) use microbes as catalysts to oxidize organicmatter at the anode and generate electricity via extracellular electrontransfer (EET) mechanisms. MFC technology development has benefited frommajor discoveries related to EET mechanisms of model organisms andcritical design evaluations, but have lacked progress in thetransformation from lab scale fundamental experiments to large scaleindustrial implementations.

While MFC technology has shown promise in the research phase, furtheradvancements are needed before MFC systems can be applied to realwastewater treatment applications. For example, early MFC research hasbeen primarily conducted in small lab-scale systems using liquid volumesless than 1 L, operated in fed-batch mode for short-term tests, or fedsynthetic or well-defined wastewater as a test solution. Yet, todemonstrate practical wastewater treatment, large-scale MFC systems areneeded to treat real wastewater under continuous flow operation overlonger time scales outside of a laboratory setting. Further,commercially viable MFC systems should have a low cost of fabrication,distribution and installment, as well as long-term operationalreliability and durability.

Disclosed are modular microbial fuel cell (MFC) devices, systems andmethods for treating wastewater and generating electrical energy througha bioelectrochemical waste-to-energy conversion process. The disclosedMFC devices, systems and methods can be used for a wide variety ofenvironmental and clean-energy applications on a large, medium or smallscale.

Implementations of the disclosed modular MFC technology can include oneor more of the following features and advantages. Example embodiments ofthe MFC devices include modular components that can be integrated into asingle, transportable casing for onsite ‘plug-and-play’ installation fora variety of end-use implementations, including wastewater treatment,sludge elimination, and electricity generation. The disclosed modularMFC technology can be used to save end-users up to 95% on wastewatertreatment operation costs based on net-zero energy operation of thedevice, remote monitoring capability, and 80% solids reduction.

Example embodiments and implementations of systems, devices and methodsin accordance with the disclosed modular MFC technology are describedherein. While some of the examples described below are primarily basedon treating domesticated animal wastewater or brewing wastewater usingcertain example configurations of the modular MFC systems, devices andmethods to facilitate understanding of the underlying concepts, it isunderstood that the disclosed embodiments can also include treatment ofother wastewater at high treatment rates from other sources in additionto farms or breweries.

FIG. 1 shows a diagram of an example embodiment of a modular MFC system100 in accordance with the present technology for net-zero energyconsuming wastewater treatment of diverse types of wastewater, includingsewage from domestic infrastructure systems, agricultural and industrialsystems. The system 100 includes a wastewater headworks system 105 toreceive raw wastewater for treatment, e.g., from a facility such as afarm, an urban sewage wastewater grid, industrial processing facility,or other, and to pre-process the raw wastewater to be provided to one ormore modular MFC devices 110, discussed further below. For example, thewastewater headworks system 105 can pre-process the raw wastewater byremoving solid or solid-like matter in the raw wastewater, e.g., of arelatively large size or mass. For example, the wastewater headworkssystem 105 can remove particles of 1 cm or greater and/or 50 g orgreater. In the example embodiment shown in FIG. 1, the wastewaterheadworks system 105 includes a degritting unit 106 (also referred to asdegritting module, degritting subsystem, or degritter) that filters outmatter of large size and/or large mass in the raw wastewater, e.g., likestones, sand, etc. In some embodiments, for example, the degritting unit106 can include a spinning device that uses centrifugal force toseparate out sand, grit and gravel, e.g., via spin filters, which can becollected and disposed as solid waste from the degritting unit 106. Insome implementations, the degritting unit 106 can include a mesh sheetto separate the solid matter from the wastewater stream. In someembodiments, the wastewater headworks system 105 includes one or moreequalization tanks 107 configured to receive and collect the degrittedwastewater and provide a steady organic load and flow of the receivedwastewater out of the wastewater headworks system 105 via apre-processed wastewater feeder box 109, e.g., to the modular MFCdevices 110. In some embodiments, the wastewater headworks system 105includes one or more chemical/physical or biological pre-treatment units108 configured to receive the degritted wastewater, e.g., from thedegritting unit 106 or the equalization tank(s) 107, and process thedegritted wastewater to remove any unwanted chemical species, such assulfur species or grease and oil for example. In some embodiments, thewastewater headworks system 105 includes a feeder unit 109 to providethe pre-processed wastewater to the next stage of the modular MFC system100. For example, the pre-processed wastewater feeder 109 can beconfigured as a feeder box. In some embodiments, the example feeder box109 is arranged at a greater height than the modular MFC devices 110.

The system 100 includes one or more modular MFC devices 110. In theexample show in FIG. 1, the system 100 includes an array of modular MFCdevices 110 that treat the pre-processed wastewater through a net-zeroenergy consumption and bioelectrochemical waste-to-energy conversionprocess generating electrical energy and digesting organic contaminantsand particulates (e.g., sludge) in the pre-processed wastewater to yieldtreated water. The bioelectrochemical conversion process implemented bythe modular MFC devices 110 includes biological oxidation accompaniedwith an extracellular transfer of electrons released during theoxidation to a conductive electrode surface/anode. The electrons arethen transferred through an external electrical circuit to a cathodewhere a reduction process occurs. The overall conversion process isspatially separated into an oxidation process via bacteria at the anodeand a reduction process at the cathode.

The example array of modular MFC devices 110 can be arranged in a seriesof 1 to N MFC devices 110, which can be referred to as a treatmenttrain. In some embodiments, the treatment train is an array ofhydraulically connected MFC devices 110 (arranged in a series of 1 to NMFC devices), of which a first MFC device 110 a receives thepre-processed wastewater from the wastewater headworks system 105,processes the pre-processed wastewater by the bioelectrochemicalconversion process and outputs the partially treated wastewater to asecond MFC device 110 b, which receives the processed output water fromthe first MFC device 110 a and processes the first MFC device 110 a'soutput water by the bioelectrochemical conversion process to output fromthe array (if N=2 MFC devices) or to another MFC device, e.g., a thirdMFC device 110 c, and so forth, until the final MFC device 110N. In someembodiments of the system 100, the pre-processed wastewater isgravity-fed from the feeder 109 of the wastewater headworks system 105to the array of modular MFC devices 110. For example, in variousimplementations, the pre-processed wastewater is gravity-fed through theutilization of a feeder box and a peristaltic pump is used at the end ofthe MFC array to control the flow rate through the MFC device(s) 110.

In some embodiments, the system 100 includes multiple treatment trains(e.g., two or more arrays of modular MFC devices 110 in series, e.g., ahydraulic series), which can treat the pre-processed wastewater inparallel to increase treatment volume. In some embodiments, the modularMFC devices 110 of a single treatment train can be arranged in one plainor in multiple vertical plains to create multiple levels. In variousembodiments, a treatment train can include the same number or adifferent number of modular MFC devices 110 as another treatment train,e.g., where treatment train 1 has N modular MFC devices 110 andtreatment train 2 has N or M modular MFC devices 110. For example, ininstances where the system 100 includes the same number of modular MFCdevices 110 among multiple treatment trains configured in parallel (froma common wastewater intake point), this can advantageously allow thesystem to distribute the wastewater processing volume load to theindividual MFC devices among each treatment train group when thetreatment trains produce similar treatment water outputs of similarqualities or level of cleanliness. Also, for example, in instances wherethe system 100 includes different numbers of modular MFC devices 110among treatment trains configured in parallel (from a common wastewaterintake point), this can advantageously allow the system to producedifferent treatment water outputs of different qualities or levels ofcleanliness.

The system 100 includes a water collection system 120 to receive thetreated water from the array of the modular MFC devices 110, which canstore the treated water and/or route the treated water to an externaldevice or system for use of the water treated by the system 100. In someembodiments, the water collection system 120 includes one or more watertanks to store the treated water, which can later be routed to theexternal device or system. In some embodiments, the system 100 includesa post-treatment system 115 arranged between the array of the modularMFC devices 110 and the water collection system 120 to post-treat thetreated water, such as disinfect the treated water or implement othertertiary treatment processes. In some embodiments, for example, thepost-treatment system 115 can include one or more membranes topost-treat the treated water using reverse or forward osmosis, advancedoxidation, denitrification, and/or disinfection, e.g., using ozone, UVlight, chlorine or other disinfection approaches.

In some example embodiments, like the system shown later in FIG. 3, themodular MFC system 100 can include twelve MFC reactors with a totalvolume of 90 L, collectively, that can be utilized to treat wastewater,e.g., such as swine wastewater at a farm, brewery wastewater at abrewery, or other types of wastewater from other types of sources. Insome example embodiments, the modular MFC system 100 can be contained ina portable container or placed on a mobile rack structure that can beeasily transported to and installed at the designated installation sitefor treatment of domestic, agricultural or industrial wastewater.

FIGS. 2A and 2B show diagrams of an example embodiment of an MFC device110 of the array of the system 100. As shown in FIG. 2A, the MFC device110 includes a bioelectrochemical reactor core 111 and a housing 119 toencase the reactor core 111. The reactor core 111 can be configured as asingle module device with the anode assembly 114 and the cathodeassembly 113. In some embodiments, the housing 119 includes a rigidcasing having a solid bottom and an opening at the top to allow modularcomponents of the reactor core 111 to be reversibly positioned withinthe interior of the housing 119. In some embodiments, the housing 119 isstructured to have a first dimension in a flow direction of thewastewater through the MFC device 110, which is configured to be longerthan a perpendicular dimension of housing 119. In the example shown inFIG. 2A, the housing 119 includes an input port 119A and output port119B arranged on opposing sides of the housing 119 along the flowdirection. The housing includes two openings 119C and 119D on opposingsides of the housing 119 that are adjacent (and perpendicular inrectangular configurations of the housing 119) to the sides having theports 119A and 119B. The openings 119C and 119D allow for air flowthrough the cathode assembly 113 encased in the housing 119. In theexample shown in FIG. 2A, the housing 119 includes an access opening ontop that is closed by an attachment plate to which the anode assembly114 is attached. In some example embodiments, the housing 119 can beclosed on top to form a whole body, in which the input port 119A and theoutput port 119B are positioned on short side walls orthogonal to thelonger dimension, and the openings 119C and 119D are positioned on longside walls orthogonal to the shorter perpendicular dimension (an exampleof which is shown in FIG. 6A).

The reactor core 111 includes a cathode assembly 113 comprising twocathode electrodes arranged on sides of the reactor proximate toopenings (e.g., openings 119C and 119D) that align with the sides of thehousing 119. The inner side of each of the cathodes is exposed to thewastewater solution fed into the reactor core 111. The reactor core 111includes an anode assembly 114 comprising a plurality of anode unitsthat are arranged between the cathode electrodes of the cathode assembly113, and which are in the flow direction between the input port 119A andoutput port 119B when the reactor core 111 is encased in the housing119.

In the anode assembly 114, for example, individual anode units can bespatially arranged and electrically connected with each other to form ananode, in which one or more anode units can operate as a single anodesystem. The anode assembly 114 can include a plurality of the singleanode systems. In some embodiments, the individual anode units can beidentical to one another within a respective single anode system. Insome embodiments, the individual anode units can be different from oneanother in a single anode system and/or to other anode units in othersingle anode systems. In some embodiments, the anode units can bevertical assemblies of anode electrodes, which can include carbon orother materials that provide surface area for microbial (e.g., bacteria)growth. The anode units of a single anode system can be electricallyconnected outside or inside the reactor core 111. Within a single anodesystem, the anode units can be connected in electrical series from anodeunit 1 to anode unit N, e.g., via wire. The wire(s) can be titanium,platinum or palladium wire, for example. In some embodiments, within theanode system, the anode units can be connected in parallel or acombination of series and parallel.

In some examples, the MFC device 110 has a rectangular shape withinternal dimensions: 31±2 cm×17±2 cm×15±2 cm, and volume of 8±3 L. Inthe example embodiment shown in FIGS. 2A and 2B, the anode assembly 114includes twenty anode units, which can be engineered as carbon anodeunits (e.g., of graphite fiber) and structured to have a 10 cm heightand 2.5 cm diameter. Other heights and diameters can also be used.

In some embodiments, at least some of the anode units of the anodeassembly 114 are electrically connected together as one electrode. Invarious implementations, for example, the anode units can be pre-treatedbefore configuration in the reactor core 111 to remove organiccontaminants or activate the anode surface, e.g., such as by soaking inan organic solvent and subsequently washing with water. Notably, forexample, based on the modular design of the reactor core 111, the anodeunits of the anode assembly 114 can be washed in such fashion afterinstallation of the MFC device 110 and the system 100 at theimplementation site.

In some examples, like the example embodiment shown in FIGS. 2A and 2B,the cathode assembly 113 includes two gas-diffusion cathodes, each of asize of 13.5 cm×27.2 cm, with geometric surface area 367 cm². Othersizes and with other surface areas may also be used. The examplegas-diffusion cathodes are placed on both sides of the reactor core 111and electrically connected. As shown in FIG. 2B, the example MFC device110 includes flow guides 112 that can be built as part of the housing119 to integrate into the reactor core 111 to direct flow of thesolution inside the reactor core 111. In some implementations, forexample, the flow guides 112 are solid static walls, which can be a partof the reactor housing 119 or additionally inserted before the placementof the reactor core 111. The flow guides 112 can be placed in a specificarrangement to direct the flow, provide proper mixing and higher protonsupply at the cathodes. The reactor of the example MFC device 110 canalso include one or more vent tubes 118A to allow gas to exit thereactor and/or one or more drain valves 118B to allow for the reactor tobe drained of liquid to allow for servicing, transporting, or removing areactor from a system.

FIG. 2C shows an illustrative diagram of a bioelectrochemicalwaste-to-energy conversion process implemented by a reactor core of anexample modular MFC device 110. As shown by this example, a biologicallycatalyzed oxidation of organic matter to dissolve carbon dioxide occursat an anode electrode of an anode unit (e.g., of the anode assembly114). The biological species performing the oxidation process include,for example, bacteria, yeast or other microorganisms. Thesemicroorganisms can be a pure culture or a whole microbial community.During the oxidation of the organic matter, the given microbes performextracellular electron transfer to the anode surface and excretion ofprotons in solution, as illustrated in the diagram of FIG. 2C. Thereleased electrons travel through an external wire to a cathodeelectrode (e.g., of a cathode plate of the cathode assembly 113).Protons diffuse from the anode surrounding to the cathode and oxygenpassively diffuses through the cathode from air. Protons, electrons andoxygen interact at the cathode surface and oxygen is reduced to hydrogenperoxide or new molecular water. The flow of electrons provides a sourceof energy generation, which can be harvested by the system 100 as one ofthe results of the bioelectrochemical process, where the other result iscleaner water that is outputted from the reactor. The modular MFCdevices 110 generate electricity as DC current, which can be stored inan electrical charge storage device (e.g., a battery). For example, anelectrical circuit configured between the anode and cathode can be usedto trickle charge batteries. In various implementations of the system100, the harvested and/or stored energy can be used to power pumps thatcan offset the power demand of system 100, e.g., such as pump 306 and/orpump 307 shown in FIG. 3.

Further example embodiments and implementations of a high-treatment ratemodular MFC system are describe, which can be employed for large-scaleapplications for wastewater treatment with net-zero energy demands,remote monitoring and high percentage of solids reduction orelimination.

In farming and livestock, for example, significant volumes of wastewaterare accumulated and discharged in the sewer during the daily washing andsanitation of animal quarters, such as pens for pig farms. During theseprocesses a vast amount of freshwater is consumed. For example,approximately one quarter of global freshwater is used for animalproduction. According to a 2011 report by the Pork Checkofforganization, a given swine farm may use 24% of its total waterconsumption for facility operations such as cooling the animals andmanure management. The most common method of manure management at swineproduction operations is to capture the wastewater in large anaerobiclagoons. However, if not properly lined or maintained, these lagoons maylead to hazardous discharge causing severe environmental impacts due tothe release of high concentrations of carbon, nitrogen and phosphorous,and the presence of potentially pathogenic bacteria. Yet, alternativetreatment methods that accelerate the removal of carbon and nitrogenfrom swine wastewater could enable water reuse for farm cleaning and/orirrigation while simultaneously preventing environmental pollution.

As described below, example results from multiple experimentalimplementations of various example embodiments of the modular MFC system100 are described. The results demonstrate and suggest the practicalutility of the disclosed MFC devices, systems and methods for wastewatertreatment and energy recovery from a variety of sources including farm,industrial and domestic wastewater, among others.

In an example implementation, an example embodiment of the modular MFCsystem 100 was operated for over 200 days in continuous mode with ahydraulic residence time of 4 hours (e.g., flow rate of 380 mL/min, 0.1gal/min) at a swine farm. Example results from this operation indicate avery stable electrochemical performance and efficient waste treatmentwith up to 65% of chemical oxygen demand (COD) removed and a maximumremoval rate of 5.0 kg-COD/m³ per day. The microbial dynamics within theMFC reactors and electrochemical performance from each reactor were alsoevaluated. These data sets suggest a very stable and robust microbialenrichment adapted to metabolize and transform a diversity of compoundspresent in highly complex wastewater into direct electricity. Further,for example, the electrochemical performance of the example modular MFCsystem 100 shows that the power recovered from the microbialtransformation of waste organics into electricity is not onlycompetitive with conventional cogeneration processes, but in-factsufficient to sustain the operational energy requirements of system 100.

FIG. 3 shows a diagram of an example embodiment of the modular MFCsystem 100, referred to as MFC system 300, used in an experimentalimplementation for continuous treatment of swine waste at a farm at flowrates, e.g., such as 660 L/day (e.g., 174 gpd) for long-term operation.The modular MFC system 300 includes twelve reactors in an array of MFCdevices 310 connected in hydraulic series with a consecutive flow of thesolution, e.g., from Reactor 1 to Reactor 2 to Reactor 3 to . . . toReactor 12. The overall volume of the example system 300 was 110 L. Thewaste stream was gravity-fed into the reactors of MFC devices 310through the utilization of a feeder box 305, and a peristaltic pump 306fluidically coupled to the array of MFC devices 310 was used toprecisely control the flow rate. For example, the reactors of the MFCdevices 310 are configured along the same plane, approximately; yet insome examples, the MFC system 300 can be configured such that the feederbox 305 is placed at a greater height than Reactor 1 of the MFC devicearray; Reactor 1 was positioned at a greater height than Reactor 2, andso forth, such that Reactor 12 had the lowest height with respect to theother reactors of the array of MFC devices 310. During continuous modeof operation, for example, the raw swine waste was stored in anunderground sump 304 (e.g., 5,700 L) and from there pumped, via a pump307, into the feeder box 305; after which, gravity-fed the reactors ofthe MFC devices 310 from the feeder box 305. In some examples, adegritter unit (not shown) is configured before the sump 304 to removethe large-mass and/or large-size particles from the raw wastewaterbefore delivered to the sump 304. The treated water,bioelectrochemically treated by the MFC devices 310, was discarded intoa sanitary sewer drain 324.

Example results and discussion of the example implementations of the MFCsystem 300 for swine wastewater treatment are described later in thisdisclosure with reference to FIGS. 9A to 22B.

FIG. 4 shows a diagram of another example embodiment of the modular MFCsystem 100, referred to as MFC system 400, used in experimentalimplementations for continuous treatment of domestic wastewater at hightreatment rates. The example modular MFC system 400 includes sixty-tworeactors in an array of modular MFC devices 410 three-dimensionallyarranged in hydraulic series within a container housing (not shown). Themodular MFC devices 410 of the array can be configured in a singlereactor unit comprising a single reactor 111, a double reactor unitcomprising two reactors 111, and/or a triple reactor unit comprisingthree reactors 111. The example MFC system 400, as shown in the diagramof FIG. 4, includes twenty-two standalone reactor units (assemblies) ofdouble and triple reactor designs that combine to provide the sixty-twototal reactors of the example MFC system 400. For example, Reactor Unit1 includes three reactors 111 (“triple reactor”) directly coupled toeach other and contained within in a single housing for hosting thetriple reactor unit; and Reactor Unit 3 includes two reactors 111(“double reactor”) directly coupled to each other and contained withinin a single housing for hosting the double reactor unit.

The array of modular MFC devices 410 are spatially arranged in thecontainer housing to begin at a height and flow the wastewater fluidthrough the array downward. This spatial arrangement allows forincreased packing density of the modular MFC devices 410 within thecontainer. The modular MFC devices 410 of the array are arranged toreceive the pre-treated wastewater from the feeder box 405 at theReactor Unit 1, e.g., via gravity-fed flow, in which the fluid undergoesa consecutive flow from Reactor Unit 1 to Reactor Unit 2 to Reactor Unit3 to . . . to Reactor Unit 22. For example, the feeder box 405 ispositioned above the plane of the reactors to provide a gravity-fedflow, e.g., in which no pumps are required for the system 400. Also, forexample, the fluid level in the feeder box 405 can affect control of theliquid level inside the reactors.

In some embodiments, for example, one or some of the modular MFC devices410 are arranged in a first vertical plane (e.g., first planar set),which output the treated fluid to the next planar set of modular MFCdevices 410 in a second vertical plane, and so forth. In someembodiments, for example, a separate feeder box can be configuredbetween the different planar sets of modular MFC devices 410. In suchimplementations, the planar feeder box for each planar level can helpprevent pressure differences and assist in gravity flow of the fluid asit passes through the array of modular MFC devices 410.

In example implementations of the system 400, the waste stream wasgravity-fed into the reactors of MFC devices 410 through the utilizationof the planar feeder boxes (note that only initial feeder box 405 isshown in the diagram). For example, the multiple vertical levels canwork as one treatment train, and the two or more levels are connectedhydraulically in series; or the two or more levels can treat thewastewater in parallel to increase treatment volume. During continuousmode of operation, for example, the waste solution can be stored in anunderground sump and from there pumped into an equalization tank (notshown). The treated water, bioelectrochemically treated by the MFCdevices 410, can be discarded into a sanitary sewer drain.

FIGS. 5A-5E show diagrams for various views of an example embodiment asingle bioelectrochemical reactor, labeled 500, in accordance with thepresent technology. In FIGS. 5A-5E, the last three digits of eachfour-digit reference numeral corresponds to one of the six sides of thereactor 500. For example, 5500 is the reference number for one of theside plates, and 5200 is the reference number for the top plate.

FIG. 5A shows a side view 5500A of the example single bioelectrochemicalreactor 500. The reactor 500 includes a reactor main housing 5000 withreactor side plate 5500 attached via fasteners 5525. The reactor sideplate 5500 has openings 5520 in the side plate to allow air to makecontact with a cathode 5560 that is permeable to air and which is placedin a plane adjacent to reactor side plate 5500 toward the inside of thereactor 500. Both sides of the reactor 500 can include cathodes that aremade from electrically conductive material and are air permeable, e.g.,air permeable cathodes. Liquid in the reactor 500 makes contact with thecathodes, the ends of the reactor, the bottom of the reactor, withanodes held into position inside the reactor, and may make contact withthe top of the reactor. On a first side of the reactor 500, a firstpermeable cathode 5560 is connected to a first side cathode contact5235; and on a second side of the reactor 500, a second permeablecathode (not shown in FIG. 5A) is connected to a second side cathodecontact. Carbon anode units 5230 are attached to the top plate 5200 ofthe reactor main housing and extend into the reactor making contact withliquid in the reactor. The reactor 500 includes a vent tube 5250 toallow gas to exit the reactor and a drain valve 5645 to allow for thereactor to be drained of liquid and/or to allow for servicing,transporting, or removing a reactor from a MFC system. In some exampleembodiments, the reactor 500 may be configured at a length (e.g., 13.26inches long) shown at 5564, and the reactor may be configured at aheight h1 (e.g., 7.5 inches tall) shown as 5557. A total heightincluding the thickness of the drain valve 5645 and the length of thevent tube 5250 is shown as h2, labeled 5555, which may be 24.93 inches.The foregoing dimensions are provided as illustrative examples of thedimensions of a reactor. Other reactors consistent with this disclosuremay be larger or smaller.

FIG. 5B shows a top view 5200A of the example single reactor 500. Carbonanode units 5230 are placed in rows along the minor axis of top plate5200. In the example of FIG. 5B, the anode units 5230 are placed in rowsof alternating length, where, for example, a first row has four anodeunits 5230, the second row has five anode units 5230, the third row hasfour anode units 5230, and a fifth row has four anode units 5230. Eachanode unit 5230 has a predetermined length and diameter with the lengthbeing longer than the diameter of the anode unit. The anode units havehigh surface area to increase the surface area of the exposure of theanodes to the liquid in the reactor. Each anode unit 5230 may beattached to the top plate 5200 via a fitting 5232 that holds the anodeunit 5230 at a predetermined height in the reactor 500, and which canprovide a liquid seal to prevent the liquid in the reactor from leakingout. Each anode unit 5230 is electrically conductive and physicallyconnected to the fitting 5232, where the anode unit 5230 is connected toa wire 5255 that electrically connects all of the anode units togetherto form the reactor anode. In some example embodiments, the electricalconnections (e.g., wires 5255) between the anode units 5230 may beenclosed inside the main reactor housing 5000. Holes 5260 in top plate5200 allow for fasteners 5265 to be inserted to attach top plate 5200 toreactor main housing 5000. Interface 5270 passes through a hole (notshown) in top plate 5200 and attaches to vent tube 5250 to allow ventingof the inside of reactor main housing 5000. Interface 5270 can include avalve allowing the build gasses to escape from the main housing 5000when interface 5270 is open.

FIG. 5C shows an end view 5300A of the example single reactor 500. Theend plate 5302 of the reactor 500 may be part of the reactor mainhousing 5000. End plate 5302 has a hole 5312 to allow for fluid to flowin/out of the reactor. The end plate at the opposite end of the reactor500 also has another hole to allow for fluid to flow out/in of thereactor. The size of the hole 5312 and the hole at the other end of thereactor may be selected to allow/restrict the flow of liquid to apredetermined flow rate which may be based on the rate at which thereactor 500 can process the liquid. Also shown in FIG. 5C are vent tube5250, vent tube interface 5270, first side cathode contact 5235, secondside cathode contact 5240 and drain 5645.

FIG. 5D shows an elevation view 5500D of the example single reactor 500.Shown in FIG. 5D are vent tube 5250, vent tube interface 5270, firstside cathode contact 5235, second side cathode contact 5240, drain 5645,fasteners 5525 and 5265, top plate 5200, end plate 5302, side plate5500, side plate openings 5520, hole 5312, and air permeable cathode5560.

FIG. 5E shows an exploded view 5500E of the example single reactor 500.Shown are reactor main housing 5000, top plate 5200, vent tube 5250,first side cathode contact 5235, second side cathode contact 5240,fittings 5232, fasteners 5265, washers 5266, and anode units 5230. Alsoshown is gasket material 5242 used to seal the top plate 5200 to thereactor main housing 5000 to prevent leakage of the liquid inside thereactor between the top plate 5200 and the reactor housing. Also shownin FIG. 5E are side plate 5500, side plate openings 5520, fasteners5525, washers 5526, inserts 5527, gasket material 5582, and one of thetwo air permeable cathodes 5560. Also shown are end plates 5300 and 5400with holes 5312 and 5412 and drain 5645.

In some example embodiments, the dimensions of the reactor include aheight of 7.5 inches (e.g., see 5557 on FIG. 5A), a length of 13.26inches (e.g., see 5564 on FIG. 5A), and a width of 8.59 inches (e.g.,see 5262 on FIG. 5B). Single reactor embodiments of other sizes may beproduced as well.

In various implementations, the single reactor performance may depend,at least in part, on the dimensions of the reactor. For example, areactor that is significantly bigger or smaller may perform less wellthan a reactor with the approximate dimensions given above. A reactorwidth (e.g., in width dimension 5262) that is too wide will increase thespacing between the cathodes and anode units in the middle of thereactor introducing high internal resistance and decreasing the protonflux from the anode units to the cathodes. For example, high internalresistance and decreased proton flux will reduce the generated currentand COD removal efficiency of the reactor. A reactor length (e.g., inlength dimension 5564) that is too long may introduce different ornonuniform flow patterns of the fluid being treated, which can causedecreased flow dynamics in the core of the reactor. For example, theliquid flow in a longer reactor is usually slower and less uniform,introducing zones in the reactor lacking proper flow dynamics. Slowerand non-uniform liquid flow also decreases current recovery and removalrates of the reactor. Notably, a smaller width of the reactor mightallow oxygen intrusion around the anode units, which impede bacterialactivity. In addition, smaller reactor treats less volume.

As such, the example single reactor 500 includes a spatial configurationof the separated cathodes (e.g., air permeable cathode plates) thatsurrounds the anode units on at least two sides that optimizes (i) thespacing (e.g., distance) between anodes and cathodes for facilitatingefficient microbial-catalyzed redox reactions, (ii) the ability forconstituent entry and diffusion (e.g., such as oxygen) into and withinthe reactor, and (iii) modularity of the reactor components for ease ofmodifications to be made, e.g., to allow custom tailoring of the reactorfor different applications and/or to allow repair.

The example single reactor 500 includes a length-to-width aspect ratioof 1.54. In some embodiments, the length-to-width aspect ratio of themodular MFC devices can be in a range of ˜1.1 to ˜2.0. The examplesingle reactor 500 includes a length-to-height aspect ratio of 1.77 anda width-to-height aspect ratio of 1.15. In some embodiments, thelength-to-height aspect ratio of the modular MFC devices can be in arange of ˜1.3 to ˜2.3; and/or the width-to-height aspect ratio of themodular MFC devices can be in a range of ˜0.8 to ˜1.5.

FIG. 6A shows a diagram depicting various views of an example embodimentof a single bioelectrochemical reactor, labeled 600. The reactor mainhousing 1100 may be produced using an injection molding or other moldingprocess. Side plates 1520 and 1120 can also be produced using injectionmolding or other molding process. Side plates 1520 and 1120 may beattached to the reactor main housing 1100 via fasteners 1525 such asscrews or other type of fastener. A gasket 1582 and an air permeablecathode 1560 may be placed between each side plate and the reactor mainhousing 1100. The gasket 1582 may be compressed by fasteners 1525 toseal the main housing and side plates to prevent fluid from leaking outfrom inside the reactor. The air permeable cathodes on each side of thereactor are electrically connected to cathode contacts 1235 and 1240.Vent tube 1250 at the top of the reactor allows for venting of gas fromthe reactor and the vent tube may include a valve to open/close the venttube. Drain 1645 allows for draining the reactor. The reactor mainhousing 1100 has a hole 1310/1610 at each end, one for fluid input andone for output. Some example embodiments may have a length L (labeled1564) of 320.6 millimeters (mm), a width W (labeled 1262) of 222.6 mm,and a height H (labeled 1555) of 231 mm (excluding an extended venttube). Other sizes of the single reactor 600 may also be produced.

The example housing structure of the single reactor 600 is specificallydesigned for cost-efficient production by an injection moldingmanufacturing process where one reactor housing 1100 is produced usingone mold. Main housing 1100 includes a first plate 1101 and a secondplate 1102, which significantly decreases the amount of manufacturingsteps and materials needed, e.g., eliminating the need of somecomponents of the single reactor 500, such as top plate 5200, fasteners5265, washers 5266 and gasket material 5242, as well as their assemblyprocess. For example, a main housing and an anode plate can bemanufactured as part of one body in reactor 600, which is produced usinga single mold. Main housing 1100 of single reactor 600 may includespecific features, in some embodiments, such as triangle ribs on thesides of the housing 1100 and rectangular ribs along the short side ofhousing 1100, which can provide physical stability and rigidity of thehousing 1100. Notably, the example side plates 1520 and 1120 shown inFIG. 6A are identical and can also be manufactured by injection moldingusing one mold. Side plates 1520 and 1120, similar to the main housingbody 1100, can be produced to have ribs for enhanced stability andrigidity.

FIG. 6B shows examples of various views of an anode and a cathode fromthe example single reactor 600. The reactor 600 can include the cathode1560, which is an air permeable cathode in this example. In someexamples, the air permeable cathode 1560 includes a carbon or otherconductive textile material with a gas-diffusion layer. In someembodiments, the cathode 1560 includes carbon powder pressed on metal orcarbon current collector. In some embodiments, the cathode 1560 caninclude a cathode with air permeable membrane to allow air permeability.In some embodiments, the cathode 1560 can include a cathode withimpregnated polymer to for air permeability and liquid resistivity. Forexample, in some embodiments, the air permeable cathode 1560 can beconfigured to have a first dimension of 279.6 mm and a second dimensionof 190 mm. The example air permeable cathode is electrically conductiveand allows air to pass through the cathode while not allowing liquid topass through the cathode. This allows for air to reach the fluid insidethe reactor while keeping the fluid in the reactor 600.

Also shown in FIG. 6B is anode plate 1200 of the reactor 600, which isintegrated with anode units 1230. The anode units may be electricallyconnected together via wires above or below anode plate 1200 and may notrequire anode fittings. Anode plate 1200 may include foldable sides 1202that are attached to the anode plate 1200 via hinges 1204. Foldablesides 1202 may be capable of being folded down to a position that isroughly parallel to the anode units 1230, which can be used as a supportfor cathodes 1560 and/or to facilitate the assembly process. Anode plate1200 may have a hole to aligned with the vent tube 1250 to allow forventing.

FIG. 6C shows an example of fitment of an anode assembly into a housingfor the example single reactor 600. The anode assembly includes anodeunits 1230 that may be electrically connected together, and anode plate1200 with foldable sides 1202A/B, shown extended, is configured to slideinto reactor main housing 1100. The anode plate 1200 may slide on ribs1205 near the inside edge of each side of the main reactor housing 1200.Ribs 1205 can be molded as part of the reactor main housing 1100. Afirst foldable side 1205A may slide along the ribs 1205 through reactormain housing 1100 to the far side where the anode assembly is centeredin the reactor main housing. Once foldable side 1202A is slid through,both foldable sides 1202A and 1202B can be folded down to a positionparallel to the anode units.

FIG. 6D shows an example of fitment of an anode assembly into a mainreactor housing for the example single reactor 600. As described abovewith respect to FIG. 6C, anode plate 1200 with foldable sides 1202A/Bextended may be slid into reactor main housing 1100, and then thefoldable sides folded down. The foldable sides include stop ribs 1207 toprevent the foldable sides from rotating beyond vertical and into themain reactor housing.

FIG. 6E shows examples of cross-sectional views of the example singlereactor 600 with the anode assembly in place. The cross-sectional viewsT-T, F-F, and R-R are identified in the view labeled 1601.

FIG. 7A shows a side view 7500A of the example double bioelectrochemicalreactor 700. Some features of the double reactor 700 are similar to somefeatures of the single reactor 500, which may be recognized by the lastthree numerals in the four-digit reference numeral of the feature. Thedouble bioelectrochemical reactor 700 includes a double reactor mainhousing 1700, which has a single wall 1702 that is shared by a firstbioelectrochemical reactor 1704 and a second bioelectrochemical reactor1706. Other than the shared wall, reactors 1704 and 1706 are bothsimilar to the single reactor 500 described in connection with FIGS.5A-5E. The shared wall 1702 has a single opening for the output of onereactor and the input to the other reactor. The first reactor 1704 andsecond reactor 1706 each include vent tubes 7250, drains 7645, anodefittings 7232, anode units 7230 (not shown), side plates 7500, fasteners7525, openings 7520, and air permeable cathodes 7560, as well as otherfeatures described in FIGS. 7A-7E.

FIG. 7B shows a top view 7200B of the example double reactor 700. Thetop of the double reactor includes vent tube interfaces 7270, fittings7232, wire 7255, and fasteners 7265. Top plate 1710 for the doublereactor may be a single plate covering the tops of both reactors 1704and 1706.

FIG. 7C shows an end view 7300A of the example double reactor 700. Theend view in FIG. 7C is similar to the end view in FIG. 5C with respectto the single reactor 500. For the double reactor 700, a fluid inputhole 1715 allows for fluid entry into the first reactor 1704. The firstreactor 1704 and second reactor 1706 each include vent tubes 7250,drains 7645, fittings 7232, anode units 7230, side plates 7500,fasteners 7525, as shown later in FIG. 7E. The vent tubes 7250, fittings7232, cathode contacts 7235 and 7240, drains 7645 and fasteners 7525 ofthe two reactors 1704 and 1706 overlap in FIG. 7C so that the elementsof the reactor furthest into the page are not visible.

FIG. 7D depicts an elevation view of a double reactor. Many of thefeatures described above in FIGS. 7A-7C and 5A-5E are shown.

FIG. 7D shows an elevation view 7500D of the example double reactor 700.Shown in FIG. 7D are vent tube(s) 7250. Also shown among FIGS. 7A-7Einclude vent tube interface 7270, first side cathode contact(s)7235/7235A, second side cathode contact(s) 7240/7240, drain 7645,fasteners 7525 and 7265, top plate 7200, end plate 7302, side plate(s)7500, side plate openings 7520, hole 1715, and air permeable cathode(s)7560.

FIG. 7E shows an exploded view 7500E of the example double reactor 700.Shown are reactor main housing 1700, top plate 1710, vent tubes 7250,first side cathode contacts 7235 and 7235A, second side cathode contacts7240 and 7240A, fittings 7232, fasteners 7265, washers 7266, and carbonanode units 7230. Also shown is gasket material 7242 used to seal thetop plate 1710 to the reactor main housing 1700 to prevent leakage ofthe liquid inside the reactor between the top plate 1710 and the reactorhousing. Also shown in FIG. 7E are side plates 7500, side plate openings7520, fasteners 7525, washers 7526, inserts 7527, gasket material 7582,and air permeable cathodes 7560, and single wall 1702 with an openingbetween reactors 1704 and 1706. The end plates have holes 1710.

In some example embodiments, the dimensions of a double reactor are 7.5inches height (from top plate to bottom plate) and 24.93 inches high(including the length of the vent tubes), by 25.62 inches long, and by8.59 inches wide. Double reactors of other sizes may be produced aswell. Reactor performance may depend, at least in part, on thedimensions of the reactor where a reactor that is significantly biggeror smaller may perform less well that a reactor with the approximatedimensions given above. In some example embodiments, the anode units andelectrical connections may be enclosed inside the main reactor housing.

The example double reactor 700 includes a length-to-width aspect ratioof 2.98. In some embodiments, the length-to-width aspect ratio of themodular MFC devices having a double reactor configuration can be in arange of ˜2.1 to ˜3.9. The example double reactor 700 includes alength-to-height aspect ratio of 3.42 and a width-to-height aspect ratioof 1.15. In some embodiments, the length-to-height aspect ratio of themodular MFC devices having a double reactor configuration can be in arange of ˜2.4 to ˜4.4; and/or the width-to-height aspect ratio of themodular MFC devices having a double reactor configuration can be in arange of ˜0.8 to ˜1.5.

FIG. 8A shows a side view 8500A of the example triple bioelectrochemicalreactor 800. Some features of the triple reactor 800 are similar to somefeatures of the single reactor 500 and/or the double reactor 700, whichmay be recognized by the last three numerals in the four-digit referencenumeral of the feature. The triple bioelectrochemical reactor 800includes a triple reactor main housing 1800, which can be configured tohave a single wall that is shared by a first bioelectrochemical reactor1804, a second bioelectrochemical reactor 1806, and a triplebioelectrochemical reactor 1808. In the example shown in FIGS. 8A-8E,the triple reactor main housing 1800 has a first wall 1802 that isshared by a first reactor 1804 and second reactor 1806, and a secondwall 1803 that is shared by the second reactor 1806 at the opposite endto 1802 and third reactor 1808. Other than the shared wall, reactors1804, 1806 and 1808 are similar to the single reactor 500 described inconnection with FIGS. 5A-5E. The shared wall 1802 has a single openingfrom the output of reactor 1804 to the input of reactor 1806 and sharedwall 1803 has a single opening from the output of reactor 1806 to theinput of reactor 1808. The first reactor 1804, second reactor 1806, andthird reactor 1808 each include vent tubes 8250, drains 8565, anodefittings 8232, anode units 8230 (not shown), side plates 8500, fasteners8525, openings 8520, and air permeable cathodes 8560, as well as otherfeatures described in FIGS. 8A-8E

FIG. 8B shows a top view 8200B of the example triple reactor 800. Thetop of the triple reactor is similar to the top of the single and doublereactors which include vent tube interfaces 8270, fittings 8232, wire8255, and fasteners 8265 as described above. Top plate 1810 for thetriple reactor may be a single plate covering the tops of all threereactors 1804, 1806, and 1808.

FIG. 8C shows an example of fitment of an anode assembly into a housingfor the example single reactor 800. The end view in FIG. 8C is similarto end views shown in FIG. 7C and FIG. 5C. A fluid input hole 1815allows for fluid entry into the first reactor 1804. The first reactor1804, second reactor 1806, and third reactor 1808 each include venttubes 8250, drains 8645, fittings 8232, anode units 8230, side plates8500, fasteners 8525. The vent tubes 8250, fittings 8232, cathodecontacts 8235 and 8240, drains 8645 and fasteners 8525 of the threereactors 1804, 1806, and 1808 overlap in FIG. 8C, and therefore theseelements of the second and third reactors furthest into the page are notvisible.

FIG. 8D shows an elevation view 8500D of the example triple reactor 800.Shown in FIG. 8D are vent tube(s) 8250.

FIG. 8E shows an exploded view 8500E of the example triple reactor 800.Shown are reactor main housing 1800, top plate 1810, vent tubes 8250,first side cathode contacts 8235, 8235A, and 8235B, second side cathodecontacts 8240, 8240A, and 8240B, fittings 8232, fasteners 8265, washers8266, and carbon anode units 8230. Also shown is gasket material 8242used to seal the top plate 1810 to the reactor main housing 1800 toprevent leakage of the liquid inside the reactor between the top plate1810 and the reactor housing. Also shown in FIG. 8E are side plates8500, side plate openings 8520, fasteners 8525, washers 8526, inserts8527, gasket material 8582, and air permeable cathodes 8560, wall 1802with an opening between reactors 1804 and 1806, and wall 1803 with anopening between reactors 1806 and 1808. The end plates have holes 1815.

In some example embodiments, the dimensions of the reactor are 7.5inches height (from top plate to bottom plate) and 24.93 inches high(including the length of the vent tubes), by 37.98 inches long, and by8.59 inches wide. Triple reactors of other sizes may be produced aswell. Reactor performance may depend, at least in part, on thedimensions of the reactor where a reactor that is significantly biggeror smaller may perform less well that a reactor with the approximatedimensions given above. In some example embodiments, the anode units andelectrical connections may be enclosed inside the main reactor housing.

The example triple reactor 800 includes a length-to-width aspect ratioof 4.42. In some embodiments, the length-to-width aspect ratio of themodular MFC devices having a double reactor configuration can be in arange of ˜3.1 to ˜5.7. The example triple reactor 800 includes alength-to-height aspect ratio of 5.06 and a width-to-height aspect ratioof 1.15. In some embodiments, the length-to-height aspect ratio of themodular MFC devices having a triple reactor configuration can be in arange of ˜3.5 to ˜6.6; and/or the width-to-height aspect ratio of themodular MFC devices having a triple reactor configuration can be in arange of ˜0.8 to ˜1.5.

Example Implementations of the MFC System 300 for Swine WastewaterTreatment

For experimentation, an example of the modular MFC system 300 wasinstalled outside proximate to a small pig farm to receive swinewastewater, and included a shade structure and monitoring system toshade and study environmental variables of temperature and humidity onthe reactors of the system. In the experimental implementations, thesystem 300 was inoculated by mixing 2.9 L of stock swine waste solution(e.g., 53,000 mg/L chemical oxygen demand (COD)); 0.5 L lagoon sedimentand 30 mM carbonate buffer, pH 7.5.

Table 1 shows the chemical composition of the swine wastewater. In Table1, COD(T) and COD(S) represent the total and soluble chemical oxygendemand, respectively.

TABLE 1 Chemical composition of swine wastewater at system inoculation.Parameter Concentration pH 7.8 COD (T), mg/L 298 COD (S), mg/L 133 NO₃⁻—N, mg/L 7 NO₂ ⁻—N, mg/L Not detected NH₄ ⁺—N, mg/L 4.3 SO₄ ²⁻, mg/LNot detected Total Suspended Solids (TSS), mg/L 530 Conductivity, mS/cm62.7 Turbidity, NTU 435

For the experimental implementation, the stock swine waste solution wasprepared from pig excrements collected from the small pig farm locatedat/near a school in Escondido, Calif. The pig excrements were mixed withtap water and blended to grind the solid waste into smallerparticulates. The swine waste suspension was then screened through astainless-steel mesh. The concentrated swine waste solution (e.g.,50,000-80,000 mg/L COD) was then added to the feeder box during batchmode and to the sump during continuous mode. The experimentalimplementation of swine wastewater treatment by the example modular MFCsystem 300 was conducted over 200 days.

For the first 30 days, the system was operated in a batch mode withrecirculation of the solution through the feeder box and the reactors ata flow rate of 1.9 L/min. The COD level during the initial 14 days ofoperation was maintained at 1000 mg/L by daily additions of stock swinewaste solution to the feeder box. The following two weeks, swine wastesolution was introduced in the system 300 once a week.

After 30 days, the operation was switched from batch to continuous flowmode. The sump was filled with swine waste solution (COD (T)˜1000 mg/L)and 30 mM carbonate buffer (pH 7.5) and was directed from the sump tothe feeder box via a cavitation pump, then through the reactors viagravity and peristaltic flow control, and discarded into the sanitarysewer drain. The flow rate was 0.38 L/min.

Each reactor was electrically monitored separately. The anode and thecathode of each reactor of the MFC device 310 were connected through aresistor which magnitude was progressively decreased from 47,000Ω to 47Ωover 30 days of operation period.

Electrochemical characterization of the bioelectrochemical treatmentprocess implemented by the reactors of the array of MFC devices 310 arediscussed below. The voltage (V) across an external resistor for eachreactor was monitored in 10 min intervals using a data logger. Thereactors were periodically disconnected to measure open circuitpotential (OCP) of the electrodes, perform polarization curves, cyclicvoltammetry (CV) measurements or electrochemical impedance spectroscopy(EIS). Polarization curves were carried out by varying the externalresistance from open circuit to 3Ω in 5 min intervals. The voltage ofthe reactor as well as the electrodes potentials were measured with eachresistor applied. Current (I) and power (P) were calculated using Ohm'slaw (I=V/R and P=V*I). The potentials of the anode and the cathode weremeasured against an Ag/AgCl reference electrode. The volumetric systemcurrent and power densities were calculated by normalizing the currentand power to the total volume of all reactors (0.09 m³).

The current and power densities of each individual reactor werecalculated as the current of the reactor normalized to the cathodesgeometric surface area (0.0734 m²). Cyclic voltammetry measurements ofthe anodes were performed by using a Potentiostat/Galvanostat Gamry 300.The potential was swept from −0.6 to 0.4 V vs. Ag/AgCl at 1 mV/s, wherethe anode was used as the working electrode and the cathode as thecounter electrode. EIS of the anode and the cathode of the reactors werecarried out at OCP from 100,000 Hz to 0.1 Hz with 5 mV applied ACvoltage.

Chemical analyses associated with the bioelectrochemical treatmentprocess implemented by the reactors of the array of MFC devices 310 arediscussed below. Chemical oxygen demand (COD), sulfate, nitrate, andnitrite of influent and effluent samples were periodically analyzedusing Hach DR850 and DR900 instruments and associated methods. TotalSuspended Solids (TSS) was quantified using EPA method 160.2. DissolvedOxygen (DO), pH, and ammonium were periodically measured on site by HachHQ40d portable meter equipped with pH, DO, and ammonium probes.

COD removal was calculated as:COD removal (mg/L)=COD_(inflow)−COD_(outflow)and COD removal efficiency (%) was calculated using the followingequation:

${{COD}\mspace{14mu}{removal}\mspace{14mu}{efficiency}\mspace{14mu}(\%)} = \frac{{COD}_{outflow}}{{COD}_{inflow} - {COD}_{outflow}}$In a similar manner, the TSS removal was determined:

TSS  removal  (mg/L) = TSS_(inflow) − TSS_(outflow)${{TSS}\mspace{14mu}{removal}\mspace{14mu}{efficiency}\mspace{14mu}(\%)} = \frac{{TSS}_{outflow}}{{TSS}_{inflow} - {TSS}_{outflow}}$The TSS loading (g/d TSS) during continuous mode of operation wascalculated as follows:TSS loading (g/d)=TSS_(inflow) (g/L)*flow rate (L/d)

The amount of produced biomass was calculated based on the TSS of thesolution (TSS_(solution)) withdrawn during regular maintenance of thesystem pipes, taking into account that the maintenance is performed onweekly bases:

${{{Biomass}\mspace{14mu}{produced}\mspace{14mu}\left( {g\text{/}d} \right)} = \frac{{TSS}_{solution}*V_{solution}}{Days}},$where V_(solution) is the volume of the solution withdrawn duringmaintenance.

Microbial composition analysis associated with the bioelectrochemicaltreatment process implemented by the reactors of the array of MFCdevices 310 are discussed below. Effluent samples were collected atvarious time-points during the experiment. In addition, before theinoculation, samples were collected from the lagoon sediment, swinewaste solution and mixed inoculum. Samples were also collected from theanode and the cathodes of reactor 6, which was decommissioned after 140days of operation. Genomic DNA was extracted from each sample using thePowerBiofilm® DNA Isolation Kit (MO Bio, Carlsbad Calif., P/N 24000-50)according to manufacturer instructions, with some minor modifications.Next, PCR was used to obtain libraries of 16S rRNA locus using theprimers 357F (5′-CCTACGGGAGGCAGCAG-3′) and 926R(5′-CCGTCAATTCMTTTRAGT-3′) and standard Illumina adapters. The ampliconlibraries were sequenced using Illumina Miseq 2×150 bp paired endtechnology. The raw reads were quality filtered and analyzed using QIIME1.0 to identify and remove chimeric sequences and perform taxonomicclassification.

Scanning electron microscopy (SEM) imaging was used in characterizationof the implementation of the system 300. During the MFC 6 decommission,small sub-sections of anodes and cathodes were collected and immediatelyimmersed in 2.5% glutaraldehyde in 1M PBS buffer and stored at 4° C.Before the SEM imaging, the samples were washed and dehydrated in 0%,10%, 25%, 50%, 75% and 100% ethanol, diluted with PBS solution asneeded. Then the samples were dried with a critical-point drier andsputtered with an Iridium layer. The coated samples were examined with aSEM (FEI XL30 SFEG) at 3 kV.

Principal Component Analysis (PCA) is a statistical tool used to analyzedata sets to find patterns. PCA visualizes correlations andanticorrelations among samples and variables. It creates uncorrelatedcomponents called principal components. The first principal component(F1) has the largest possible variance and the second, orthogonal to thefirst, has the largest possible inertia (F2). PCA in XLSTAT (Addinsoft)was applied to a dataset of samples collected from the decommissioningof reactor 6.

Canonical correspondence analysis (CCA) was performed using XLSTAT todescribe the correlations between community composition andenvironmental factors. CCA is a comparative evaluation tool that canvisualize correlations between key environmental variables andassociated species (phylotype) compositions.

Example results of the experimental implementation of the system 300 isdescribed below.

Example Results for Electrochemical Performance and Characterization,and COD Removal Rate

After inoculation, each reactor was connected by a 47,000Ω resistor. Theimmediate response of the reactors was a voltage of ˜0.1V, which is mostlikely due to the accumulated electrochemically active compounds in theswine wastewater or stored charge in bacteria. After the initialdischarge and drop, the voltage of each reactor gradually increased to˜0.3 V at day 2 to 0.6 V at day 6. The start-up time of the system 300is short (e.g., less than 24 hours). A fluctuation in the generatedvoltage following the day and night cycles can also be seen.

During the enrichment phase the system was operated under batch modewith daily feeds to maintain a relatively constant level of COD (e.g.,˜1000 mg/L). One example goal of this phase of the experimentalimplementation was the development of a robust and functionallyselective microbial community at the anode surface, to prepare thesystem 300 for continuous flow through. Therefore, to provide enoughenergy for bacterial growth and biofilm development, the reactors wereinitially connected through a high resistor of 47,000Ω, which wasswitched to 4,700Ω during the second week. The following resistors of330Ω and 47Ω were selected based on cell polarization measurements wherethe most efficient resistor was the one corresponding to a powerslightly lower than the maximum power from the polarization curves.After week two, an overpotential was applied to the electrodes toprovide the selective pressure for electroactive bacteria. However,applying a resistor of 47Ω led to fast decrease in the cathodicpotential and the system was placed back to 330Ω at the end of the batchmode, and remained under this resistor during continuous mode.

FIGS. 9A and 9B show data plots depicting current density for eachindividual reactor when normalized to the cathodes surface area (FIG.9A) and the reactor volume (FIG. 9B).

The generated current was used as an indicator of the performance ofeach reactor (FIGS. 9A and 9B). The reactors demonstrated similarelectrochemical performance. For example, the average current density atday 100 was 14.9±1.4 mA/m² (186±10 mA/m³), with a relative standarddeviation (RSD) of 5%, indicating very good reproducibility andidentical reactor performance. No trend was observed in ascending ordescending current along the series of reactors.

The maximum average current density of 103±7 mA/m² (1011±73 mA/m³) wasachieved under 47Ω resistor and corresponds to 37±5 mW/m² (362±52mW/m³). For example, this lower electrochemical characteristic of thedesigned reactors herein might be due to the markedly lower CODloadings.

FIG. 10 shows a data plot depicting open circuit potential (OCP)measurements of the anode and the cathodes for reactor 5 of the exampleMFC system 300. The OCP of the separate electrodes was also monitored ona regular basis. FIG. 10 shows the OCP of the anode (e.g., 20 anodeunits operated as a single anode system), and left and right cathodes ofreactor 5 as a representative reactor. As can be seen in the data plot,the anode developed a stable electrochemical potential in 35 days, whichstayed constant for the remainder of the operation. At the same time theOCPs of the two cathodes decreased from 183±6 mV and 182±7 mV to −33±40mV and −22±17 mV for the left and right cathodes, respectively.

For example, the sharp decrease in the cathodic potential was likely dueto the higher polarization at 47Ω although the system showed the highestenergy recovery at this point. Therefore, the resistance was switchedback to 330Ω and remained at this value for the rest of the study.Prolonging the cathodes operation was of a major importance since inmost of the long-term studies reported to-date, the cathodes were theelectrodes that failed over time and required repair or replacement.Also, one example goal of the experimental study using the system 300was organics removal and wastewater treatment, not maximized energyharvesting. Thus, operating the system 300 with sub-optimal energygeneration conditions was acceptable for the example implementations.

FIGS. 11A and 11B show data plots depicting polarization and powercurves (FIG. 11A) and polarization curves for the cathode and anode ofreactor 5 during batch mode of operation (FIG. 11B). The electrochemicalperformance of the reactors increased during batch mode due to thegradual anode development (FIGS. 11A and 11B). The maximum powerincreased from 18 to 105 mW/m² (133 to 1067 mW/m³) from day 14 to day30, accompanied by a 6.5 times enhancement of the generated current,from 79 mA/m² (800 mA/m³) at day 14, to 533 mA/m² (5,200 mA/m³) at day30. Although the OCP of the anode did not change significantly (−533±6mV vs. Ag/AgCl) during batch mode, the ability of the electrogeniccommunity to participate in charge transfer with the anode surface wasdramatically increased.

The cathodic potential decreases from day 14 to day 23 were most likelydue to the development of a biofilm at the cathode surface. It is alsonotable that the overall electrochemical output of the reactor wasdetermined by the anode performance during batch mode of operation.

FIGS. 12A-12D show data plots depicting polarization (FIGS. 12A, 12C)and power curves (FIG. 12B) as well as maximum power development overtime (FIG. 12D) for reactor 5 of the example MFC system 300 duringcontinuous mode. Different behavior was observed when the system 300 wasoperated under continuous flow. During continuous mode, the reactorsshowed power output in the range of 84-105 mW/m² (800-933 mW/m³) at339-379 mA/m² (3333-4000 mA/m³) from day 30 until day 150 when the CODloading was decreased to 500-600 mg/L COD(T). Due to the lower organiccontent the electrochemical output decreased to 53-64 mW/m² (533-667mW/m³) of power at approximately 273 mA/m² (2667 mA/m³).

The Michaelis-Menten constant for this study was 1,510 mg/L COD(T),which is higher than the COD loading till day 150, and three timeshigher than the COD loading after day 150 in this study. Therefore, adecrease of the electrochemical output of the MFCs can be expected as afunction of the decreased COD loading.

A constant and slight decay in the current was recorded after day 150because of the decreased electrode performance as seen from thepolarization curves of the anode and cathode of reactor 5, e.g., shownin FIG. 12C. Over time the anodic potential became less negative, butthe anode still possessed stable polarization. The cathode suffered fromhigher potential losses than the anode and in general dictated thegenerated current.

FIG. 12D shows the maximum power for reactor 5 of the example system300, determined through cell polarization and power curves. During batchmode, the maximum power of the reactor was low due to the undevelopedanode. After it reached day 30, the maximum power stayed around 92±8mW/m² (905±78 mW/m³) until day 142, after which it decreased ultimatelyto 53 mW/m² (519 mW/m³) measured day 198. In general, higher P_(max) wasrecorded at higher COD loading rates, which is expected since thecurrent and power are extracted from the oxidation of the organicmaterial.

The internal resistance of the reactor determined from the slope of thepolarization curves was 100Ω at day 14, 22Ω at day 23 and stayed 10Ωfrom day 30 to day 200 (e.g., shown in FIG. 12D). Therefore, theelectrochemical performance of the reactors was not influenced byvariations or an increase of the reactors' internal resistance. Theaccumulation of biomass in the reactors and at the electrode surfacesdid not lead to an increase in R_(int), which indicates that thereaction rates at the two electrodes were not changed during operation.

The decreased cathodic output is most likely a result of decreasedoxygen diffusion through the cathodes due to biofilm formation or saltsaccumulation. In our study, a pronounced decline in the cathodicoperation was not observed (FIG. 12C) although a thick biofilm wasdeveloped on the cathode surface. Notably, formation of struviteprecipitate on the cathode surface facing the solution can occur atbasic pH, which is assumed to occur near the cathode surface due togenerated hydroxide as a byproduct of oxygen reduction. Notably, noprecipitate formation was visually observed in this study.

FIG. 13 shows a data plot depicting an EIS of a reactor of the examplesystem 300 at different time intervals. EIS of the anode was used toevaluate the charge transfer resistance at the anode (FIG. 13). Thestarting anodic charge transfer resistance was 0.1Ω and graduallyincreased to 1.2Ω at day 205. The charge transfer resistance at theinitial stages and at day 205 were very low, which indicates highlyactive and fast bacteria-electrode interactions, e.g., rapidextracellular electron transfer.

The solution resistance also increased slightly from 1.2Ω to 9.4Ω due todecreased conductivity of the solution. The solution was no longerbuffered after day 150, and the COD loading decreased from 1000 mg/L to500 mg/L after day 110. In addition, biomass was built up in thereactors, which also leads to decreased conductivity and increased ohmicresistance.

Example Results for Chemical Analysis of Wastewater Composition

Samples for chemical analysis of the inflow and outflow of the exampleMFC system 300 were taken periodically to evaluate the COD removal rateas a main parameter, as well as to determine the ability of the systemto remove nitrogen and sulfur-containing inorganic pollutants.

FIGS. 14A and 14B show data plots depicting COD concentration of theinfluent, effluent and the COD removal for the experimentalimplementation (FIG. 14A) and COD treatment rate over time (FIG. 14B).During the first 110 days, the target COD for the inflow of the MFCsystem 300 was 1000 mg/L, after which it was decreased to approximately500 mg/L. FIGS. 14A and 14B show that on average, the higherinstantaneous COD removal in mg/L was observed during the initial stageof continuous mode, when the COD loadings were higher. The organicremoval rate of the system varied in the range of 1-5 kg/m³.

The COD removal rate constant was calculated based on the assumption offirst order rate constant as: k(h⁻¹)=−ln(COD_(inflow)/COD_(outflow))/HRT, where COD_(inflow) is theinfluent COD, COD_(outflow) is the effluent COD and HRT is 4 hours. TheCOD removal rate constant at COD_(inflow)˜1000 mg/L was 0.112±0.07 h⁻¹,and at COD_(inflow)˜500 mg/L, k=0.135±0.07 h⁻¹. The rate constants atthe two COD loadings were not significantly different at P=0.01, whichindicates that the COD removal in this study followed a first orderreaction rate, and the rate of COD removal is dependent on theconcentration of inflow COD.

The maximum COD treatment rate of z 5.0 kg COD/m³ per day was observedat day 95, where the COD loading was 2200 mg/L. The maximum COD removalefficiency (65%) was seen on day 102. The lowest COD removal wasrecorded during the enrichment period when the system was under batchmode.

FIG. 15 shows a data plot depicting COD removal efficiency (%) duringcontinuous mode of system operation. The COD removal efficiency was onaverage lower at higher COD loading, and was more consistent and stablewhen the COD loading was decreased. In the example implementations,36±15% COD removal efficiency was interested under continuous mode andHRT of 4 h (as shown in FIG. 15), and significant solids sedimentationwas not observed in the reactors over the course of operation. Thehighest COD removal efficiency achieved by the example MFC system 300was 65% at different time points of operation.

An MFC used for odor removal demonstrated 84% COD removal under batchmode for 260 h. The control MFC, kept under open circuit, already showed53% COD removal because of anaerobic fermentation. Therefore, it can beassumed that the COD removal due to the operation of the MFC as abioelectrochemical system is only 31% of the overall observed removal,which is comparable to the average removal rates in this example study.

The example MFC system 300 in this study has a significantly higherworking volume and faster flow rate than conventional system of previousstudies, which is important for practical wastewater treatment. Forexample, the COD removal efficiency of the MFC system 300 in the exampleimplementations is one of the highest among the MFCs running undercontinuous mode; the current and power densities (per square cm) of theMFC system 300 in this study are comparable to other studies; and thenet energy recovery (NER) observed in this study is significantly higherthan other systems. The normalized energy recovery (NER) of the MFCsystem 300 at 330Ω and continuous mode of operation was 0.11 kWh/kg COD,which is higher than the NER of anaerobic digestion treatment plant withenergy recovery from methane. Due to the lower COD removal rates duringbatch mode, the NER was calculated as 0.22 kWh/kg COD with CoulombicEfficiency (CE) of 27%. On average, the CE of the MFC system 300 undercontinuous mode was estimated as 7%.

Nitrogen and sulfur are major contaminants in wastewater and theirremoval is also a key parameter for sustainable wastewater treatmenttechnology. Nitrogen can be removed from wastewater through biologicalnitrification and denitrification steps. Nitrification involves theoxidation of ammonium to nitrate with the participation of oxygen andnitrifying bacteria. Nitrate can be further reduced to nitrite andultimately to nitrogen gas. The electrons necessary for thedenitrification process can be provided by the oxidation of organicmaterial.

Denitrification (Scheme 1) can be performed in solution or by using theelectrons captured at the anode and transferred to the cathode wherenitrate reduction will appear, e.g., (E^(o′) _(NO3/NO2)=+433 mV vs. SHE,E^(o′) _(NO2/NO)=+350 mV vs. SHE, E^(o′) _(NO3/N2)=+700 mV vs. SHE). Inan MFC, for example, the denitrification reaction can typically occur atthe cathode surface via (i) direct electron transfer from the cathode tomicroorganisms or (ii) intermediate production of H₂, which is furtherused by bacteria as an electron donor for nitrate reduction. A competingreaction to denitrification reaction is the dissimilatory nitratereduction to ammonium (DNRA) (Scheme 1).

FIGS. 16A-16C show data plots depicting concentration of NO₃ ⁻—N (FIG.16A), NO₂ ⁻—N (FIG. 16B) and NH₄ ⁺—N (FIG. 16C) in the influent (in) andeffluent (out) of the system during continuous mode. Note: *—theconcentration in the influent and effluent was zero.

The two possible processes for conversion of nitrate in an MFC aredenitrification to nitrogen gas or DNR. Due to the anaerobic conditionsin MFC reactors, nitrification of ammonium does not proceed, thereforeit is accumulated in the effluent. The latter process is what was seenin some of the implementations of the example system 300. As such, DNRAis an example main pathway for nitrate removal in the MFC system 300(e.g., example results shown in FIGS. 16A-16C). When nitrate is presentin the influent it is rapidly reduced to ammonium. The nitrate removalefficiency was 60-100% under continuous mode (e.g., FIG. 16A). Nitriteis usually not present in swine wastewater or it is not produced inmeasurable amounts during the treatment process (e.g., FIG. 16B).

In the samples, the concentration of ammonium increased when thewastewater passed through the reactors. An increase in the ammoniumcontent was observed to be 30-40% on average, and up to 90% was recorded(e.g., FIG. 16C). The latter is a clear indication of DNRA, for example.During heterotrophic nitrate reduction, DNRA can occur in solution at ahigh C/N ratio due to the high organic content, which translates intohigh electron donor content.

The example MFC system 300 is a bioelectrochemical system with high C/Nratio, thus both mechanisms, heterotrophic and autotrophic, of DNRA canproceed and lead to the production of ammonium. In the exampleimplementations, the C/N ratio, calculated as COD/NO₃ ⁻—N, for theexample system 300 was determined to be in the range of 28 to 380. Theexample results from the microbial composition analysis of the cathodespopulation showed the presence of Rhodocyclales, which are known asaerobic, denitrifying bacteria (FIG. 20). Geobacter lovleyi was alsofound at the cathode surface in small abundance (e.g., <0.5%) and hasbeen identified as a DNRA-capable species. As a result of the neutral pHin the reactors, ammonium does not transform into ammonia andaccumulates in the effluent.

It should be noted that there is not an EPA regulation for ammoniumconcentrations; however, taste and smell limitations are in the range of35 mg-N/L to 0.2 mg-N/L, respectively. Nitrate and nitrite water qualitylimitations are 10 mg-N/L and 1 mg-N/L, respectively. The example MFCsystem 300 showed ammonium removal and a decrease of the nitrate andnitrite concentrations, which provides treated water that can meet andexceed these thresholds.

Swine waste is not characterized with high sulfate content. The highestamount of sulfate measured in a concentrated swine waste solution was250 mg/L and most of the time sulfate was not detectable. Sulfateremoval up to 70% was demonstrated in the example MFC system 300 (e.g.,shown in FIG. 17).

FIG. 17 shows a data plot depicting sulfate concentration in theinfluent (in) and effluent (out) of the system 300 during continuousmode. Note: *—the concentration of sulfate in the influent and effluentwas zero. Sulfate was reduced to sulfide, and sulfide levels did notexceed 6 mg/L. Sulfate is regulated as a secondary contaminate, with amaximum contamination level of 250 mg/L. The influent and effluent fromthe MFC did not exceed this value over the course of operation.

One of the main disadvantages of conventional aerobic wastewatertreatment technologies is the significant production of biomass, whichrequires additional sludge management. For swine wastewater, forexample, the level of total suspended solids (TSS) is higher thandomestic wastewater and ranges from 400-500 mg-TSS/L versus less than100 mg-TSS/L for domestic wastestreams. For 4 hours of HRT, the TSSamount decreased by 50 to 80% with the production of biomass less than0.12% of the initial TSS loading. It was estimated that 270±32 g/d ofTSS are introduced into the system with 1.7 g/d build up as a biomass inthe reactors.

One of the concerns of two-chamber MFCs is the accumulation of protonsin the anodic chamber causing pH to become acidic, and the depletion ofprotons and accumulation of hydroxide in the cathodic chamber, whichleads to a basic environment. In a single chamber MFC, this problem isusually not observed but even in this example design the MFC's solutionwas still buffered for a time. A 33 mM carbonate buffer was used tomaintain constant and close to neutral pH (e.g., shown in FIG. 18).However, after day 150, the swine wastewater was not buffered but pHstill remained relatively constant. DO of the inflow was also constantlymonitored to ensure anaerobic conditions and the DO measured wasconsistently below the detection limits of the probe.

FIG. 18 shows a data plot depicting pH and DO profiles over time.

Example Results for Microbial Composition Analyses

Microbial composition and dynamics of the example MFC system 300 weremonitored and evaluated by 16S rRNA sequence analysis in theexperimental implementations. The inoculum source and system outflowduring batch and continuous modes of operation were surveyed as well asthe anode and cathode associated populations for a single reactor.

FIG. 19 shows a plot depicting a microbial community profile of theexample MFC system effluent using 16S rRNA sequences. Samples fromlagoon sediment, swine waste stock solution and the mixed inoculum werecollected at the time of system inoculation (FIG. 19). As seen, themicrobial population in the inoculum was mainly determined by themicrobial diversity and relative abundance of the stock swine wastesolution. A high relative abundance of fermentative bacteria from theorders of Bacteroidales and Clostridiales were observed in the inoculumand the system solution at day 0. These two orders occupied nearly 80%of the community. However, after day 4, the relative abundance ofBacteroidales and Clostridiales decreased to roughly 20% and 15% of thetotal community population, respectively, which remained relativelystable in the effluents throughout 136 days of operation.

Although Desulfuromonadales were not present in detectable numbers inthe inoculum, they became a relatively abundant population in the systemsolution starting at day 58. Most Desulfuromonadales bacteria observedin solution were from the genus of Geobacter and the species identifiedwere associated to Geobacter lovleyi. The abundance of G. lovleyicorrelates with the DNRA reactions observed in our system at day 58onwards, as stated previously.

After day 1, the relative abundance of fermentative Pseudomonadalessignificantly increased and remained broadly present in the solutionduring batch and the beginning of continuous mode of operation; however,the relative abundance of Pseudomonadales notably decreased duringcontinuous mode. This initial increase and later decrease coincide withCOD content and removal rate in the system, which, for example, mayreinforce the notion that efficient fermenters, such as those from orderPseudomonadales, contribute to performance of waste water treatment inMFCs. The persistence of Pseudomonadales under the batch mode may alsobe attributed to their ability to form persistent biofilms that aredifficult to remove. In addition, species in the genus Pseudomonas arecapable of degrading complex aromatic compounds, giving them a uniqueniche in the microbial consortium enriched during the time when CODcontent was relatively high, which likely provided complex organics assubstrates.

Species in the order Flavobacteriales were apparent around day 50 anddisappeared later. It is unclear whether the changed electrochemical andbiochemical environment at the onset of continuous mode of operationplayed a deterministic role in enriching flavobacteria, which are knownto be aerobic or facultative anaerobic chemoorganitrophs with bothrespiratory and fermentative metabolic capacities. The example data fromthe example implementations were insufficient to predict the causativelinks between their relative abundance and one or more abiotic or bioticfactors.

Campylobacterales appear during batch mode and became one of thedominant orders during continuous mode especially around day 58.Campylobacterales are common inhabitants of gastrointestinal tracts inruminant animals as well as humans. Most campylobacterales arefastidicous and are adapted pH below neutral. In the example MFC system300, the onset of fluctuations in relative abundance ofCampylobacterales is correlated with the change in pH, which drops belowneutral around day 58, and again around day 150, e.g., coinciding withmajor shifts in the abundance of Campylobacterales. Also,campylobacterales are known to release sulfur and iron into theirenvironment, thereby affecting pH due to the addition of sulfur, as wellas promoting growth of bacteria that are dependent on sulfur compoundsas substrates for growth and metabolism.

Bacteria from the orders Desulfovibrionales and Desulfobacterales, knownas sulfate reducing bacteria, were seen in a higher abundance duringcontinuous mode, especially after day 58, coinciding with the increasingabundance of Campylobacterales. The appearance of these sulfate reducingphylotypes also correlates well with removal of sulfate from the exampleMFC system 300 observed around the same time.

When samples were collected from the feed stock inflow, and outflow ofeach reactor (day 50 and/or day 58), no significant differences inmicrobial composition or relative abundance were observed. The microbialpopulation in the effluent samples from all twelve reactors wasidentical at the order-level, consistent with the comparableelectrochemical performance of the 12 reactors in the example MFC system300. Further, no significant changes in relative abundances wereapparent at the order-level when comparing the microbial composition ofthe inflow wastewater and the outflow composition from each reactorduring continuous mode. These data suggest that the microbial populationin solution are not significantly impacted, at the order-level, by a4-hour retention time in the example MFC system 300.

It was also evidenced in the example implementations, by the performedcyclic voltammetry measurements, that the shifts in microbialcomposition affect the anodic performance. The onset of the oxidationreaction shifts to more negative potentials over time accompanied withan increase in the generated oxidation current.

While community differentiation in each reactor was not observed in thesystem effluents, it is possible that a longer hydraulic retention timeor additional reactors in the treatment train would induce a uniquemicrobial selection. It is also possible that a higher resolutionanalysis at the genus- or species-level may provide a deeper insightinto unique microbial compositions in the twelve reactors. Microbialdifferentiation may also be more apparent in the electrode-associatecommunities.

To evaluate the microbial populations associated with the anodes,cathodes, and residual sludge, a single reactor (reactor 6) wasdecommissioned. Reactor 6 had similar performance to all other reactorsat the time of sampling (day 105). Samples were extracted from eachanode brush, at the top and bottom. Six samples were also extracted pereach left and right cathode at different locations of flow (in, middle,and outflow) as well as the top and the bottom of each section (e.g.,shown in FIG. 20). The microbial composition on the anode brushes werenearly identical at the order-level in terms of diversity and relativeabundance indicating an adequate water flow and mixing creating ahomogenous environment in our reactors.

FIG. 20 shows a plot depicting a 16S rRNA-based microbial communityprofile of samples collected from the anode and cathodes of reactor 6 ofthe example MFC system 300 after its decommissioning. The numberindicates the brush from where the sample was collected and T means thetop of the brush, B is the bottom of the brush. LC and RC are left andright cathodes; in, mid and out indicate the sampling position in termsof flow direction, and T and B are the top and bottom of the cathodes.

The most relatively abundant bacteria at the anode belonged to theorders of Clostridiales, Bacteroidales, which likely contributedfermentative capacity. Desulfovibrionales, which is a sulfate reducinggroup, and Rhodocyclales, which catalyze versatile set of biochemicalreactions including denitrification in both aerobic and anaerobicenvironments were also observed. For example, because the anodiccommunities were reproducible across reactors over time and performance,the stable co-existence of fermentative and electroactive species inthese communities provide strong evidence that electroactive enrichmentscan be selected and maintained from a variety of waste streams forpractical applications. It was also observed that the microbialcommunities at the anode surfaces were distinctly different incomposition and relative abundance when compared to the thosecharacterized from the effluent solution and inflow. This also confirmsthe stability of biofilm enrichments and associated function in fielded,pilot-scale microbial fuel cells, for example.

The cathode microbial communities were also distinctly different inmicrobial composition and relative abundance when compared to the anodesand inflow communities. However, the microbial composition of the leftand right cathodes, at different sampling points on the surface, werevery similar to each other even though the relative abundances ofdifferent orders were apparent. The dominant microbial populationsincluded bacteria from the order Burkholderiales due to the presence ofoxygen.

FIGS. 21A and 21B show SEM images showing biofilm on the anode (FIG.21A) and the cathode (FIG. 21B) of reactor 6 of the example MFC system300.

The SEM images of the anode and cathode subsamples from reactor 6confirmed the presence of diverse microbial communities, which was morepronounced on the cathode surface, likely due to the presence of oxygenat the gas diffusion electrodes. In the example implementations, thebiofilm on the anode brushes was not uniform and was mainly observed inbetween fibers of the brushes. It is hypothesized, for example, that thesurface of graphite brushes is very smooth and hydrophobic, and preventsthe formation of a strongly attached biofilms. Notably, for example,camera images taken during reactor decommissioning showed the presenceof thick and uniform biofilm on the brushes, which apparently is lostduring sample processing for SEM due to its loose attachment to thegraphite fibers. The cathodic surface was entirely covered with a thickbiofilm as evidenced by both camera and SEM images (e.g., FIG. 21B). Thepresence of bacteria appendages in between cells and significantextracellular material can be observed for both anode and cathodebiofilms.

Example Results Including Statistical Analysis

Canonical Correspondence Analysis (CCA) was performed on the effluentdata collected from the system over time. These analyses enable thecharacterization of possible correlations between reactor operationvariables and the associated microbial diversity and abundance.

FIGS. 22A and 22B show data plots including a CCA biplot of samplescollected from the system effluent (FIG. 22A), and a PCA biplot ofsamples collected during the decommissioning of reactor 6 (FIG. 22B).

The CCA biplot showed that inflow as well as effluent microbialcommunity during batch mode had a higher abundance of Pseudomonadalesand Spirochaetales, which disappear over time in continuous mode ofoperation. It was established that over time reactors were enriched withsulfate reducing (Desulfobacteriales and Desulfuromonadales) anddenitrifying bacteria (Rhodocyclales). The latter were mainly found atthe anode (e.g., FIG. 22B) and aerobic bacteria from the orderBurkholderiales were predominant at the cathode surface due to thepresence of oxygen. Burkholderiales have been identified inacetate-amended, denitrifying microbial communities, showing thatdenitrification might occurred at the cathodes. The Burkholderialesorder are also one of the most common bacterial orders found in watersystems and some species have been reported as opportunistic pathogens.

Principal Component Analysis is a statistical tool used in the last 5years for data analysis of microbial fuel cell experiments. For example,it has advantages in visual representation of correlations and easy datainterpretation, especially for large datasets multiple variables, whichoften requires multi-dimensional scaling techniques. The samplescollected from the anode brushes and the cathodes of reactor 6 after ithad been decommissioned were analyzed by PCA (e.g., FIG. 22B). Thesamples from the anode brushes were clustered together with a slightseparation between top and bottom. The bottom of the brushes waspopulated with more species from the order Clostridia. Also, Clostridiaat the anode appeared together with sulfate reducing bacteria andelectrogenic bacteria (Desulfuromonadales).

A synergistic interaction exists between Clostridia and sulfate reducingbacteria. Fermentative bacteria also cooperate with electrogenicbacteria in MFCs. During symbiotic cooperation, the fermentativebacteria break down more complex organic compounds to volatile fattyacids, which are then used by sulfate reducing bacteria or electrogenicbacteria such as Geobacter spp.

The example implementations of the system 300 demonstrated the designand operation of a large scale MFC system for continuous treatment ofswine wastewater at a small farm with an HRT of 4 hours. The subset ofdata presented herein include only 210 days of operation, and, notably,the system 300 is capable to currently operate. The example resultsshowed maximum current density during this time frame as 103±7 mA/m²(1011±73 mA/m³) and corresponds to 37±5 mW/m² (362±52 mW/m³). Thenormalized energy recovery (NER) of the MFC system at 330Ω was 0.11kWh/kg COD, which is higher to the NER of anaerobic digestion treatmentplant with energy recovery from methane. The maximum COD treatment rateof ˜5.0 kg COD/m³ per day was observed at day 95, where the COD loadingwas 2200 mg/L. The maximum COD removal efficiency (65%) was seen on day102.

Example Implementations of the MFC System 400 for Domestic WastewaterTreatment

FIG. 23 shows images of an example implementation of the modular MFCsystem 400. The example MFC system 400, which is illustrated in FIG. 4,was used in experimental implementations for continuous treatment ofdomestic wastewater at high treatment rates.

For example, the modular MFC system 400 was installed inside a 20-footshipping container and placed in proximate to a small residentialneighborhood to receive domestic wastewater. The MFC devices 410 of thesystem 400 were arranged in two vertical plains, top and bottomtreatment trains. Each vertical plane of MFC devices 410 was forming asingle treatment train. In the experimental implementations, the twotreatment trains were inoculated by mixing domestic wastewater (e.g.,550 mg/L chemical oxygen demand (COD)); 31 L lagoon sediment and 30 mMcarbonate buffer, pH 7.5.

For the experimental implementation, the modular MFC devices 410 of thetop treatment train were labeled from 1 to 11 and included double andtriple reactors 111. Each reactor in the double and/or triple reactorMFC device were names as A, B and C following the flow direction. Forexample, Reactor Unit 1 includes three reactors 111 directly coupled toeach other and contained within in a single housing for hosting thetriple reactor design. The first reactor 111 is designated as A, thesecond reactor as B and the third reactor as C. Reactor Unit 3 includestwo reactors 111 directly coupled to each other and contained within ina single housing for hosting the double reactor design. The firstreactor was designated as A and the second reactor 111 was designated asB.

For the experimental implementation, the MFC devices 410 of the bottomtreatment train were labeled from 12 to 22 and included double andtriple reactors 111. Each reactor in the double and/or triple MFC devicewere names as A, B and C following the flow direction.

Table 2 shows the chemical composition of the domestic wastewater. InTable 2, COD(T) and COD(S) represent the total and soluble chemicaloxygen demand, respectively.

TABLE 2 Chemical composition of domestic wastewater at systeminoculation. Parameter Concentration pH 7.3 COD (T), mg/L 550 COD (S),mg/L 392 NO₃ ⁻—N, mg/L 25.4 NH₄ ⁺—N, mg/L 21.1 SO₄ ²⁻, mg/L 300Conductivity, mS/cm 62.7 Sulfide, mg/L 7.9

For the experimental implementation, a separate feeder boxes were usedfor the planar sets of modular MFC devices 410. The feeder box for eachplanar level help prevent pressure differences and assist in gravityflow of the fluid as it passes through the array of modular MFC devices410.

For the first 30 days, the system was operated in a batch mode withrecirculation of the solution through the feeder box and the reactors ata flow rate of 1.9 L/min. The COD level was maintained at 500 mg/L byweekly media exchanges of system 400, when new domestic wastewater wasintroduced.

Each reactor 111 of the modular MFC device(s) 410 was electricallymonitored separately. The anode and the cathode of each reactor 111 ofan MFC device 410 were connected through a resistor which magnitude wasprogressively decreased from 47,000Ω to 1,000Ω over 30 days of operationperiod.

Electrochemical characterization of the bioelectrochemical treatmentprocess implemented by the reactors of the array of MFC devices 410 arediscussed below.

Example Results for Electrochemical Performance and Characterization,and COD Removal Rate

After inoculation, each reactor was connected by a 47,000Ω resistor. Theimmediate response of the reactors was a voltage of ˜0.3V, whichgradually increased to ˜0.5 V at day 2 and 0.67 V at day 4. The start-uptime of system 400 was short (e.g., less than 24 hours) as previouslyobserved with system 300.

During the enrichment phase the system was operated under batch modewith weekly media exchanges to maintain a relatively constant level ofCOD (e.g., ˜500 mg/L). For example, a media exchange was done bydraining half of the treatment train solution and replacing it with newraw domestic wastewater. The latter increased the COD loading of thesystem.

FIGS. 24A and 24B show data plots depicting current density for eachindividual reactor when normalized to the cathodes surface area for toptreatment train (FIG. 24A) and bottom treatment train (FIG. 24B).

The generated current was used as an indicator of the performance ofeach reactor (FIGS. 24A and 24B). The maximum current densitiesgenerated by the reactors of the top treatment train were 4.0±1.2 mA/cm²with Relative Standard Deviation (RSD) of 30%, which is higher than theRSD of the current densities for the reactors of the bottom treatmenttrains. The reactors of the bottom treatment train demonstrated maximumcurrent densities of 1.9±0.2 mA/cm² and RSD=9%. For example, the higherdeviation of the current densities for the top train reactors is due toa replacement of the cathodes for some reactors, which led to highercurrent densities generated by them.

In general, it can be concluded that the reactors from a given treatmenttrain performed similarly in terms of current generation. No trend wasobserved in ascending or descending current along the series ofreactors.

FIG. 25 shows a data plot depicting open circuit potential (OCP)measurements of the anode and the cathodes for reactor 7B of the exampleMFC system 400. The OCP of the separate electrodes was also monitored ona regular basis. FIG. 25 shows the OCP of the anode (e.g., 20 anodeunits operated as a single anode system), and left and right cathodes ofreactor 7B as a representative reactor. As can be seen in the data plot,the anode developed a stable electrochemical potential in 15 days, whichstayed constant for the remainder of the operation. At the same time theOCPs of the two cathodes decreased from 154 mV to 0 mV for bothcathodes.

Example Results for Chemical Analysis of Wastewater Composition

Samples for chemical analysis of the example MFC system 400 were takenbefore and after the batch cycles and from the inflow and outflow duringcontinuous mode of operation. COD removal rate as a main parameter wasevaluated and the ability of the system to remove nitrogen andsulfur-containing inorganic pollutants was also determined.

FIG. 26 show data plots depicting COD removal efficiency of the exampleMFC system 400 under batch mode. For example, the COD removal efficiencyfor the top treatment train was 53±16% and for the bottom treatmenttrain, the COD removal efficiency was 29±11%. The higher removalefficiency of the top train is a result of the better electrochemicalperformance of the reactors from the top treatment train.

The organic removal rate of the overall system 400 during continuousmode was in the range of 21 to 47%.

It is noted from the example implementations that the example MFC system400 used in the implementation had a higher working volume than MFCsystem 300, but COD removal efficiency of the MFC system 400 in theexample implementations is comparable to MFC system 300 described above.The normalized energy recovery (NER) of the MFC system 400 at 1000Ω andcontinuous mode of operation was 0.24 kWh/kg COD, which is higher thanthe NER of MFC system 300 due to the higher number of reactors. Due tothe lower COD removal rates during batch mode, the NER was calculated as9.8 kWh/kg COD with Coulombic Efficiency (CE) of 31%. On average, the CEof the MFC system 400 under continuous mode was estimated as 32%.

FIG. 27 show data plots depicting concentration of NO₃ ⁻—N and NH₄ ⁺—Nin the system during batch mode. Similar to the example MFC system 300,the example MFC system 400 treating domestic wastewater also convertsnitrate into ammonium following DNRA pathway for nitrate removal. Whennitrate is present in the influent it is rapidly reduced. The nitrateand nitrite removal efficiencies under continuous mode were 70% and 20%,respectively.

In the samples, the concentration of ammonium increased when thewastewater passed through the reactors. An increase in the ammoniumcontent was observed to be 6, which also indicates that along DNRA, thenitrate and nitrite are reduced to nitrogen gas. The latter is highlydesirable reaction in wastewater treatment and nitrite/nitrate removal.

It should be noted that there is not an EPA regulation for ammoniumconcentrations; however, taste and smell limitations are in the range of35 mg-N/L to 0.2 mg-N/L, respectively. Nitrate and nitrite water qualitylimitations are 10 mg-N/L and 1 mg-N/L, respectively. The example MFCsystem 400 showed ammonium removal and a decrease of the nitrate andnitrite concentrations, which provides treated water that can meet andexceed these thresholds.

FIG. 28 shows a data plot depicting sulfate concentration of the system400 during batch mode. Sulfate was reduced to sulfide during thetreatment process. The accumulation of sulfide in the system can cause adecrease in the cathodic potential due to catalyst poisoning and/orcompetitive cathodic sulfate reduction. A pretreatment step forsulfate/sulfide removal is necessary to be implemented in the treatmentprocess.

FIG. 29 shows a data plot depicting pH profile over time. The pH of thesystem was maintained neutral throughout operation.

Example Implementations of the MFC System 3000 for Brewery WastewaterTreatment

FIG. 30 shows a diagram of another example embodiment of the modular MFCsystem 100, labeled MFC system 3000, used in experimentalimplementations for continuous treatment of brewery wastewater at hightreatment rates and flow rates, e.g., such as 570 L/day (e.g., 150 gpd)for long-term operation. The example modular MFC system 3000 includes anarray of modular MFC devices 3010 arranged in hydraulic series on amobile rack structure. The modular MFC devices 3010 of the array can beconfigured in a single reactor design, a double reactor design, and/ortriple reactor designs. The example MFC system 3000, as shown in thediagram of FIG. 30, includes twelve standalone reactor assemblies ofsingle reactor designs.

The array of modular MFC devices 3010 are spatially arranged in onevertical plane and a single treatment train. The modular MFC devices3010 of the array are arranged to receive the pre-treated wastewaterfrom a feeder box 3005 at a Reactor 1, e.g., via gravity-fed flow, inwhich the fluid undergoes a consecutive flow from Reactor 1 to Reactor 2to Reactor 3 to . . . to Reactor 12. For example, the feeder box 3005 ispositioned above the plane of the reactors to provide a gravity fedflow.

In some embodiments, for example, the fluid level in the reactors iscontrolled by a U-loop configuration or assembly of pipes with a shapeof reverse U. In such embodiments of the system 3000, the outflow of thearray of modular MFC devices 3010 flows through the U-loop before itreaches a collection tank 3030. The U-loop can be positioned above theplane of the array of modular MFC devices 3010 and below the plane ofthe feeder box 3005. The fluid levels between feeder box 3005 and U-loopdetermines the liquid flow through MFC devices 3010.

In some embodiments, for example, the raw wastewater is provided byequalization tank 3020 and the treated effluent is discharged into acollection tank 3030. In some embodiments, for example, the MFC system3000 includes a sulfur removal unit 3040, which assists in thepre-treatment of the raw wastewater.

While double and triple reactor configurations are shown in the examplemodular MFC devices 3010 of the system 3000 in FIG. 30, the modular MFCdevices 3010 can include other combinations of single, double and/ortriple bioelectrochemical reactors. Examples of the single reactors arefurther detailed in connection with FIGS. 5A-5E and FIGS. 6A-6E;examples of the double reactors are further detailed in connection withFIGS. 7A-7E, and examples of the triple reactors are further detailed inconnection with FIGS. 8A-8E. Various components of the single, double,and/or triple reactors, e.g., such as the reactor housings, may be madeusing a computer numerical control (CNC) machine or may be machined byanother process by hand or using other machine tool.

In an example experimental implementation, the example modular MFCsystem 3000 was installed outside a small brewery to receive brewerywastewater, and included a shade structure and monitoring system toshade and study environmental variables of temperature and humidity onthe reactors of the system. In the experimental implementations, thesystem 3000 was inoculated by mixing brewery wastewater (e.g., 26,000mg/L chemical oxygen demand (COD)); 0.5 L lagoon sediment and 30 mMcarbonate buffer, pH 7.5.

Table 3 shows the chemical composition of the brewery wastewater. InTable 3, COD(T) and COD(S) represent the total and soluble chemicaloxygen demand, respectively.

TABLE 3 Chemical composition of brewery wastewater at systeminoculation. Parameter Concentration pH 5.88 COD (T), mg/L 26.560 COD(S), mg/L 26.480 NO₃ ⁻—N, mg/L 11 NO₂ ⁻—N, mg/L Not detected NH₄ ⁺—N,mg/L 21 SO₄ ²⁻, mg/L Not detected S²⁻, mg/L Not detected Total SuspendedSolids (TSS), mg/L 246 Conductivity, mS/cm 1.46 Volatile Fatty Acids(VFA), mg/L 355 Protein, mg/L 232

For the experimental implementation, the brewery was collected from asmall brewery at Joshua Tree, Calif. The brewery wastewater was used asit is. The brewery wastewater was then added to the equalization tankand fed into the feeder box. The experimental implementation of brewerywastewater treatment by the example modular MFC system 3000 wasconducted over 250 days.

The system was operated mainly in a batch mode with recirculation of thesolution through the feeder box and the reactors at a flow rate of 0.38L/min. A raw brewery wastewater was introduced in the system 3000 once aweek.

The system was periodically switched into continuous mode for CODremoval rates evaluation. Under continuous mode, the brewery wastewaterwas flowing from the equalization tank to the feeder box, through thereactors and collected in a collection tank.

Each reactor was electrically monitored separately. The anode and thecathode of each reactor of the MFC device 3010 were connected through aresistor which magnitude was progressively decreased from 47,000Ω to200Ω during operation.

Electrochemical characterization of the bioelectrochemical treatmentprocess implemented by the reactors of the array of MFC devices 3010 arediscussed below. The voltage (V) across an external resistor for eachreactor was monitored in 30 min intervals. The reactors wereperiodically disconnected to measure open circuit potential (OCP) of theelectrodes.

The current and power densities of each individual reactor werecalculated as the current of the reactor normalized to the cathodesgeometric surface area (0.0734 m²).

Example results of the experimental implementation of the system 3000are described below.

Example Results for Electrochemical Performance and Characterization,and COD Removal Rate

After inoculation, each reactor was connected by a 47,000Ω resistor andslowly decreased to 200Ω. The voltage of each reactor graduallyincreased to ˜0.3 V at day 2 to 0.6 V at day 6. The start-up time of thesystem 3000 is short (e.g., less than 24 hours). A fluctuation in thegenerated voltage following the day and night cycles can also be seen.

FIG. 31 show data plot depicting current density for each individualreactor when normalized to the cathodes surface area. The generatedcurrent was used as an indicator of the performance of each reactor. Thereactors demonstrated relatively similar electrochemical performance.The maximum average current density of 25 mA/m² was achieved under 200Ωresistor and corresponds to 9 mW/m².

FIG. 32 shows a data plot depicting open circuit potential (OCP)measurements of the anode and the cathodes for reactor 11 of the exampleMFC system 3000. The OCP of the separate electrodes was also monitoredon a regular basis. FIG. 32 shows the OCP of the anode (e.g., 20 anodeunits operated as a single anode system), and left and right cathodes ofreactor 11 as a representative reactor. The anodic OCP is highlydependent on the COD loading where a higher COD resulted in morenegative anodic potential. The cathodic potential slowly decreased overtime from 100 mV to −300 mV vs. Ag/AgCl. The decreased cathodicpotential is most likely a result of the specific brewery wastewatercomposition and the occurrence of competitive reactions.

Example Results for Chemical Analysis of Wastewater Composition

Samples for chemical analysis of the inflow and outflow of the exampleMFC system 3000 were taken periodically to evaluate the COD removal rateas a main parameter, as well as to determine the ability of the systemto remove nitrogen and sulfur-containing inorganic pollutants.

FIGS. 33A and 33B show data plots depicting the COD removal rate as mg/LCOD removed (FIG. 33A) and COD removal efficiency (FIG. 33B) for theexperimental implementation under batch mode. FIGS. 34A and 35B show theCOD removal rate as mg/L COD removed (FIG. 34A) and COD removalefficiency (FIG. 34B) for the experimental implementation undercontinuous mode.

On average, for the example implementations, the higher COD removal inmg/L under batch mode was observed during the initial stages when theCOD loadings were higher. The average COD removal rate under batch modewas 300 mg/L and the average COD removal efficiency was 18%.

On average, for the example implementations, the higher instantaneousCOD removal in mg/L under continuous mode was 2800 mg/L and COD removalefficiency of 25%. The average COD removal rate under continuous modewas 1230 mg/L and the average COD removal efficiency was 12%.

The normalized energy recovery (NER) of the MFC system 3000 at 400Ω andcontinuous mode of operation was 2.8 kWh/kg COD, which is higher thanthe NER of anaerobic digestion treatment plant with energy recovery frommethane. Due to the lower COD removal rates during batch mode, the NERwas calculated as 4.5 kWh/kg COD with Coulombic Efficiency (CE) of 54%.On average, the CE of the MFC system 3000 under continuous mode wasestimated as 20%.

EXAMPLES

In some embodiments in accordance with the present technology (exampleA1), a microbial fuel cell (MFC) system for wastewater treatmentincludes a wastewater headworks module to receive and pre-treat rawwastewater for feeding pre-treated wastewater from the wastewaterheadworks module; one or more modular MFC devices tobioelectrochemically process the pre-treated wastewater thatconcurrently generates electrical energy and digests organiccontaminants and particulates in the wastewater to yield treated water,the one or more modular MFC devices including a bioelectrochemicalreactor and a housing to encase the bioelectrochemical reactor, whereinthe bioelectrochemical reactor includes a plurality of anode unitsarranged between a cathode assembly; and a water collection module toreceive the treated water from the one or more modular MFC devices andstore the treated water and/or route the treated water from the system.

Example A2 includes the system of any of the preceding or subsequentexamples, wherein the wastewater headworks module includes a degrittingmodule to filter solid matter having one or both of a large size andmass.

Example A3 includes the system of example A2, wherein the degrittingmodule includes at least one of a spinning apparatus or a mesh apparatusto separate and collect the matter for disposal as solid waste from thesystem.

Example A4 includes the system of example A2, wherein the wastewaterheadworks includes one or more equalization tanks to receive and collectthe degritted wastewater and modulate a steady organic load and flow ofthe pre-treated wastewater.

Example A5 includes the system of example A2, wherein the wastewaterheadworks includes one or more pre-treatment modules to remove unwantedchemical species including one or more of sulfur species, grease or oil.

Example A6 includes the system of any of the preceding or subsequentexamples, wherein the housing of the one or more modular MFC devicesincludes a rigid casing having a solid bottom and an opening at a top ofthe housing to allow modular components of the bioelectrochemicalreactor to be positioned within and removed from an interior of thehousing.

Example A7 includes the system of example A6, wherein the housing has afirst dimension in a flow direction of fluid through the modular MFCdevice.

Example A8 includes the system of example A7, wherein the housingincludes an input hole and output hole arranged on opposing sides of thehousing along the flow direction.

Example A9 includes the system of example A7, wherein the housingincludes a first opening and a second opening on opposing sides of thehousing perpendicular to the flow direction that allow for air flowthrough the cathode assembly of the bioelectrochemical reactor.

Example A10 includes the system of any of the preceding or subsequentexamples, wherein the anode units each include carbon fibers thatprotrude from an interior cylinder.

Example A11 includes the system of any of the preceding or subsequentexamples, wherein the cathode assembly includes two gas-diffusioncathodes.

Example A12 includes the system of any of the preceding or subsequentexamples, wherein the one or more modular MFC devise include an array ofa plurality of the modular MFC devices arranged in a hydraulic series,wherein a first modular MFC device receives the pre-treated wastewaterfrom the wastewater headworks module and bioelectrochemically processesthe pre-treated to output a first treated water received at a secondmodular MFC device of the array, which bioelectrochemically processesthe first treated water to output a second treated water received at anext modular MFC device of the array.

Example A13 includes the system of example A12, wherein the array of theplurality of the modular MFC devices are arranged in the hydraulicseries in multiple vertical planes between one or more groups of modularMFC devices of the array.

Example A14 includes the system of any of the preceding or subsequentexamples, wherein the one or more modular MFC devices include a doublereactor configuration including two of the bioelectrochemical reactorsencased in the housing, and/or wherein the one or more modular MFCdevices include a double reactor configuration including three of thebioelectrochemical reactors encased in the housing.

Example A15 includes the system of any of the preceding or subsequentexamples, wherein the pre-treated wastewater is gravity-fed through afeeder box to at least one of the one or more modular MFC devices.

Example A16 includes the system of example A15, further comprising aperistaltic pump coupled at the end of the one or more modular MFCdevices or at the end of a series of an array of the modular MFC devicesto control a flow rate of the fluid through the modular MFC devices.

Example A17 includes the system of any of the preceding or subsequentexamples, wherein the wastewater includes sewage received from adomestic infrastructure system, agricultural system, or industrialsystem.

Each of the above examples can include a modular microbial fuel cell(MFC) device, system and/or method for treating wastewater andgenerating electrical energy through a bioelectrochemicalwaste-to-energy conversion process.

In some embodiments in accordance with the present technology (exampleB1), a microbial fuel cell (MFC) device for bioelectrochemicallyprocessing wastewater includes a fluidic input port for receiving thewastewater; an MFC housing; a plurality of anode units suspended in ahousing to contact the wastewater; one or more cathode electrodes,wherein the one or more cathode electrodes are gas permeable andelectrically conductive; and a fluidic output port for outputtingtreated wastewater.

Example B2 includes the MFC device of the preceding or subsequentexamples, wherein the MFC device generates electrical energy andprocesses organic contaminants and particulates in the wastewater toyield treated water.

Example B3 includes the MFC device of the preceding or subsequentexamples, wherein the fluidic input port and the fluidic output port arearranged on opposing sides of the housing along a direction of flow.

Example B4 includes the MFC device of example B3, wherein the directionof flow is perpendicular to an air flow through the cathode assembly ofthe bioelectrochemical reactor, and wherein the air flow enablesprocessing of the wastewater in the reactor.

Example B5 includes the MFC device of the preceding or subsequentexamples, wherein the MFC housing includes a rigid casing having a solidbottom and openings at a top and sides of the MFC housing to allowmodular components of the bioelectrochemical reactor to be positionedwithin and removed from an interior of the MFC housing.

Example B6 includes the MFC device of the preceding or subsequentexamples, wherein the MFC housing has a first dimension in a flowdirection of fluid through the MFC device, a second dimension orthogonalto the first dimension and the one or more cathode electrodes, and athird dimension orthogonal to the first dimension and the seconddimension, and wherein the first dimension lies in a range of 13.26inches±0.5 inches, the second dimension lies in a range of 8.59inches±0.5 inches, and the third dimension lies in a range of 7.5inches±0.5 inches.

Example B7 includes the MFC device of the preceding or subsequentexamples, wherein the one or more cathode electrodes are gas-diffusionelectrodes.

Example B8 includes the MFC device of the preceding or subsequentexamples, wherein the one or more cathode electrodes are electricallyconnected together.

Example B9 includes the MFC device of the preceding or subsequentexamples, wherein each of the plurality of anode units compriseconductive branches positioned radially around a central anode core,wherein the central anode core is parallel to the one or more cathodeelectrodes.

Example B10 includes the MFC device of the preceding or subsequentexamples, wherein the anode unit branches comprise carbon.

Example B11 includes the MFC device of the preceding or subsequentexamples, wherein the anode units of the plurality of anode units areelectrically connected together external to the housing.

Example B12 includes the MFC device of the preceding or subsequentexamples, wherein the MFC housing is produced by computer numericalcontrolled machining.

Example B13 includes the MFC device of the preceding or subsequentexamples, wherein the housing is produced by injection molding.

Example B14 includes the MFC device of the preceding or subsequentexamples, wherein the plurality of anode units are electricallyconnected inside the housing.

In some embodiments in accordance with the present technology (exampleB15), a microbial fuel cell (MFC) system for wastewater treatmentincludes a wastewater headworks module to receive wastewater; and one ormore MFC devices to bioelectrochemically process the wastewater, whereineach MFC device includes an MFC housing, a plurality of anode unitssuspended in the housing to contact the wastewater, and one or morecathode electrodes, wherein the one or more cathode electrodes are gaspermeable, and a water collection module to receive the treated waterfrom the one or more MFC devices.

Example B16 includes the MFC system of the preceding or subsequentexamples, wherein the MFC system is a double reactor comprising two MFCdevices fluidically connected in series, wherein a first MFC devicereceives the wastewater form the wastewater headworks module, wherein alast MFC device provides the treated water to the water collectionmodule, and wherein first treated water from the first MFC device isprovided as input water to the last MFC device.

Example B17 includes the MFC system of example B16, wherein the MFChousing for the double reactor has a first dimension in a flow directionof fluid through the MFC device, a second dimension orthogonal to thefirst dimension and the one or more cathode electrodes, and a thirddimension orthogonal to the first dimension and the second dimension,and wherein the first dimension lies in a range of 25.62 inches±0.5inches, the second dimension lies in a range of 8.59 inches±0.5 inches,and the third dimension lies in a range of 7.5 inches±0.5 inches.

Example B18 includes the MFC system of the preceding or subsequentexamples, wherein the MFC system is a triple reactor comprising threeMFC devices fluidically connected in series, wherein a first MFC devicereceives the wastewater form the wastewater headworks module, whereinfirst treated water from the first MFC device is provided as secondinput water to the second MFC device and a second treated water from thesecond MFC device is provided as third input water to a third MFCdevice, and wherein the third MFC device provides the treated water tothe water collection module.

Example B19 includes the MFC system of the preceding or subsequentexamples, wherein the MFC housing for the triple reactor has a firstdimension in a flow direction of fluid through the MFC device, a seconddimension orthogonal to the first dimension and the one or more cathodeelectrodes, and a third dimension orthogonal to the first dimension andthe second dimension, and wherein the first dimension lies in a range of37.98 inches±0.5 inches, the second dimension lies in a range of 8.59inches±0.5 inches, and the third dimension lies in a range of 7.5inches±0.5 inches.

Example B20 includes the MFC system of the preceding or subsequentexamples, wherein the wastewater includes sewage received from adomestic infrastructure system, agricultural system, or industrialsystem.

Each of the above examples can include a modular microbial fuel cell(MFC) device, system and/or method for treating wastewater andgenerating electrical energy through a bioelectrochemicalwaste-to-energy conversion process.

In some embodiments in accordance with the present technology (exampleC1), a system for energy generation and wastewater treatment includes awastewater headworks system to pre-treat raw wastewater by removingsolid particles and produce a pre-treated wastewater that that isoutputted from the wastewater headworks system; one or more modularmicrobial fuel cell (MFC) devices to bioelectrochemically process thepre-treated wastewater by concurrently generating electrical energy anddigesting organic contaminants and particulates in the pre-treatedwastewater to yield a treated water, the one or more modular MFC devicescomprising a housing and a bioelectrochemical reactor that is encasedwithin the housing, wherein the bioelectrochemical reactor includes aplurality of anodes arranged between a cathode assembly; and a watercollection system to receive the treated water from the one or moremodular MFC devices and store the treated water and/or route the treatedwater from the system.

Example C2 includes the system of example C or any of the subsequentexamples among C3-C20, wherein the wastewater headworks system includesa degritting device to filter and/or remove solid matter having one orboth of a large size and mass.

Example C3 includes the system of example C2, wherein the degrittingdevice includes at least one of a spinning apparatus or a mesh apparatusto separate and collect the filtered solid matter for disposal as solidwaste from the system.

Example C4 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the wastewater headworks system includesone or more equalization tanks to receive and collect the degrittedwastewater and modulate a steady organic load and flow of thepre-treated wastewater.

Example C5 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the wastewater headworks system includesone or more chemical, physical or biological pre-treatment devices toremove unwanted chemical species including one or more of sulfurspecies, grease or oil.

Example C6 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the housing of the one or more modularMFC devices includes a rigid casing having a solid frame that allowsmodular components of the bioelectrochemical reactor to be positionedwithin and removed from an interior of the housing.

Example C7 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the modular MFC device has a firstdimension in a flow direction of fluid through the modular MFC devicethat is larger than a second dimension perpendicular to the firstdimension.

Example C8 includes the system of examples C7 and/or C9, wherein thehousing includes an input hole and output hole arranged on opposingsides of the housing along the flow direction.

Example C9 includes the system of examples C7 and/or C8, wherein thehousing includes a first opening and a second opening on opposing sidesof the housing perpendicular to the flow direction that allow for airflow into the bioelectrochemical reactor to provide oxygen to thecathode assembly.

Example C10 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein an anode of the plurality of anodesincludes carbon fibers that protrude from an interior cylinder.

Example C 11 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the cathode assembly includes twogas-diffusion cathodes separated on two sides of the plurality of anodesand arranged longitudinally along a flow direction of fluid through thebioelectrochemical reactor, the gas-diffusion cathodes able to allowoxygen to permeate into the fluid within the bioelectrochemical reactor.

Example C 12 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the one or more modular MFC devicesinclude an array of a plurality of the modular MFC devices arranged in ahydraulic series, wherein a first modular MFC device receives thepre-treated wastewater from the wastewater headworks system andbioelectrochemically processes the pre-treated wastewater to output afirst treated water that is received at a second modular MFC device ofthe array, which bioelectrochemically processes the first treated waterto output a second treated water received at a next modular MFC deviceof the array.

Example C13 includes the system of example C12, wherein the array of theplurality of the modular MFC devices are arranged in the hydraulicseries in multiple vertical planes between one or more groups of modularMFC devices of the array.

Example C14 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the one or more modular MFC devicesinclude a double reactor configuration, the double reactor configurationincluding two of the bioelectrochemical reactors encased in the housingof the one or more modular MFC devices.

Example C15 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the one or more modular MFC devicesinclude a triple reactor configuration, the triple reactor configurationincluding three of the bioelectrochemical reactors encased in thehousing of the one or more modular MFC devices.

Example C16 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the pre-treated wastewater is gravity-fedthrough a feeder box to at least one of the one or more modular MFCdevices.

Example C17 includes the system of any of the preceding or subsequentexamples among C1-C20, further comprising a peristaltic pump coupled atthe end of the one or more modular MFC devices or at the end of a seriesof an array of the modular MFC devices to control a flow rate of thefluid through the one or more modular MFC devices or the array of themodular MFC devices.

Example C18 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the modular MFC devices are operable toclean the pre-treated wastewater and yield the treated water under aflow rate of at least 100 mL/min and/or at least 120 L/day, and/or whichcan be operated for at least 10 consecutive hours of operation.

Example C19 includes the system of any of the preceding or subsequentexamples among C1-C20, wherein the one or more modular MFC device isoperable to generate the electrical energy and produce the treated waterwith net-zero energy consumption.

Example C20 includes the system of any of the preceding examples amongC1-C19, wherein the raw wastewater includes sewage received from adomestic infrastructure system, agricultural system, or industrialsystem.

In some embodiments in accordance with the present technology (exampleC21), a method for energy generation and wastewater treatment includespretreating a raw wastewater by removing at least some solid particlesfrom a wastewater fluid that produces a pre-treated wastewater;processing the pre-treated wastewater by a bioelectrochemical conversionprocess that generates electrical energy and concurrently cleans thepre-treated wastewater to produce treated water by digesting matter inthe wastewater fluid; extracting the generated electrical energy forstorage or transfer to an external electrical device; and outputting thetreated water.

Example C22 includes the method of example C21 or any of the subsequentexamples among C23-C35, wherein the bioelectrochemical conversionprocess includes: oxidizing organic matter in the wastewater fluid bybiologically-catalyzed oxidation using biological species within abioelectrochemical reactor comprising a plurality of anodes spatiallyarranged between at least two cathodes, which causes excretion ofprotons in solution that diffuse to a cathode in the bioelectrochemicalreactor and an extracellular transfer of electrons released during theoxidation to a conductive surface of an anode in the bioelectrochemicalreactor, and electrically transferring the electrons to the cathodethrough an electrical circuit, which facilitates electrochemicalreduction at the cathode by interaction of the protons, the electronsand oxygen to produce hydrogen peroxide and/or new water.

Example C23 includes the method of example C22, wherein the biologicalspecies include at least one of a bacteria or a yeast.

Example C24 includes the method of any of the preceding or subsequentexamples among C21-C35, in which the method includes collecting thepre-treated wastewater in one or more equalization tanks and modulatinga steady organic load and flow of the pre-treated wastewater to thebioelectrochemical reactor.

Example C25 includes the method of any of the preceding or subsequentexamples among C21-C35, wherein the outputting the treated waterincludes one or both of storing the treated water in a tank and routingthe treated water to an external fluidic device.

Example C26 includes the method of any of the preceding or subsequentexamples among C21-C35, wherein the pretreating the raw wastewaterincludes removing the at least some of the solid particles having one orboth of a large size and mass.

Example C27 includes the method of any of the preceding or subsequentexamples among C21-C35, wherein the pretreating the raw wastewaterincludes removing unwanted chemical species including one or more ofsulfur species, grease or oil from the wastewater fluid.

Example C28 includes the method of any of the preceding or subsequentexamples among C21-C35, wherein the processing the pre-treatedwastewater by the bioelectrochemical conversion process is implementedin a microbial fuel cell (MFC) device comprising a bioelectrochemicalreactor encased within a housing, the bioelectrochemical reactorcomprising a plurality of anodes spatially arranged between at least twocathodes.

Example C29 includes the method of example C28 and/or any of thepreceding or subsequent examples among C21-C35, wherein an anode of theplurality of anodes includes carbon fibers that protrude from aninterior cylinder.

Example C30 includes the method of example C28 and/or any of thepreceding or subsequent examples among C21-C35, wherein the cathodeassembly includes two gas-diffusion cathodes separated on two sides ofthe plurality of anodes and arranged longitudinally along a flowdirection of fluid through the bioelectrochemical reactor, thegas-diffusion cathodes able to allow oxygen to permeate into the fluidwithin the bioelectrochemical reactor.

Example C31 includes the method of example C28 and/or any of thepreceding or subsequent examples among C21-C35, wherein the modular MFCdevice is included in an array of a plurality of the modular MFC devicesarranged in a hydraulic series, wherein a first modular MFC devicereceives the pre-treated wastewater and bioelectrochemically processesthe pre-treated wastewater to output a first treated water that isreceived at a second modular MFC device of the array, whichbioelectrochemically processes the first treated water to output asecond treated water outputted from the array or received at a nextmodular MFC device of the array.

Example C32 includes the method of example C28 and/or any of thepreceding or subsequent examples among C21-C35, wherein the pre-treatedwastewater is gravity-fed to the modular MFC device.

Example C33 includes the method of any of the preceding or subsequentexamples among C21-C35, wherein the processing the pre-treatedwastewater to generates the electrical energy and concurrently producesthe treated water occurs under a flow rate of at least 100 mL/min and/orat least 120 L/day, and/or which can be operated for at least 10consecutive hours of operation.

Example C34 includes the method of any of the preceding or subsequentexamples among C21-C35, wherein the processing the pre-treatedwastewater by the bioelectrochemical conversion process generates theelectrical energy and produces the treated water with net-zero energyconsumption.

Example C35 includes the method of any of the preceding examples amongC21-C34, wherein the raw wastewater includes sewage received from adomestic infrastructure system, agricultural system, or industrialsystem.

In some embodiments in accordance with the present technology (exampleC36), a device for energy generation and wastewater treatment includes amodular microbial fuel cell (MFC) device operable tobioelectrochemically process wastewater that includes organic matter ina fluid that concurrently generates electrical energy and digests theorganic matter to yield a treated water, the modular MFC devicecomprises: a housing, and a bioelectrochemical reactor encased withinthe housing, the bioelectrochemical reactor including a plurality ofanodes arranged between a cathode assembly, wherein the cathode assemblyincludes two gas-diffusion cathodes separated on two sides of theplurality of anodes and arranged longitudinally along a flow directionof the fluid through the bioelectrochemical reactor, the gas-diffusioncathodes able to allow oxygen to permeate into the fluid within thebioelectrochemical reactor.

Example C37 includes the device of any of the preceding or subsequentexamples among C36-C47, wherein the housing includes a rigid casinghaving a solid frame that allows modular components of thebioelectrochemical reactor to be positioned within and removed from aninterior of the housing.

Example C38 includes the device of any of the preceding or subsequentexamples among C36-C47, wherein the modular MFC device has a firstdimension in the flow direction that is larger than a second dimensionperpendicular to the first dimension.

Example C39 includes the device of any of the preceding or subsequentexamples among C36-C47, wherein the housing includes an input hole andoutput hole arranged on opposing sides of the housing along the flowdirection.

Example C40 includes the device of any of the preceding or subsequentexamples among C36-C47, wherein the housing includes a first opening anda second opening on opposing sides of the housing perpendicular to theflow direction that allow for air flow into the bioelectrochemicalreactor to provide oxygen to the cathode assembly.

Example C41 includes the device of any of the preceding or subsequentexamples among C36-C47, wherein an anode of the plurality of anodesincludes carbon fibers that protrude from an interior cylinder.

Example C42 includes the device of any of the preceding or subsequentexamples among C36-C47, wherein the modular MFC device includes a doublereactor configuration, the double reactor configuration including twobioelectrochemical reactors arranged in series along the flow directionand encased in the housing.

Example C43 includes the device of any of the preceding or subsequentexamples among C36-C47, wherein the modular MFC device includes a triplereactor configuration, the triple reactor configuration including threebioelectrochemical reactors arranged in series along the flow directionand encased in the housing.

Example C44 includes the device of any of the preceding or subsequentexamples among C36-C47, wherein the modular MFC devicebioelectrochemically processes the wastewater: oxidizing the organicmatter in the wastewater fluid by biologically-catalyzed oxidation usingbiological species within the bioelectrochemical reactor, which causesexcretion of protons in solution that diffuse to a cathode of thecathode assembly and an extracellular transfer of electrons releasedduring the oxidation to a conductive surface of an anode of theplurality of anodes, and electrically transferring the electrons to thecathode through an electrical circuit, which facilitates electrochemicalreduction at the cathode by interaction of the protons, the electronsand oxygen to produce hydrogen peroxide and/or new water.

Example C45 includes the device of any of the preceding or subsequentexamples among C36-C47, wherein the modular MFC device is operable tobioelectrochemically process the wastewater under a flow rate of atleast 100 mL/min and/or at least 120 L/day, and/or which can be operatedfor at least 10 consecutive hours of operation.

Example C46 includes the device of any of the preceding or subsequentexamples among C36-C47, wherein the modular MFC device is operable togenerate the electrical energy and produce the treated water withnet-zero energy consumption.

Example C47 includes the device of any of the preceding examples amongC36-C46, wherein the wastewater includes sewage received from a domesticinfrastructure system, agricultural system, or industrial system.

In some embodiments in accordance with the present technology (exampleC48), a device for energy generation and wastewater treatment includes afirst modular microbial fuel cell (MFC) device and a second modular MFCdevice. The first modular MFC device is operable to bioelectrochemicallyprocess wastewater that includes organic matter in a fluid thatconcurrently generates electrical energy and digests the organic matterto produce a treated water, and the first modular MFC device comprises:a first housing, and a first bioelectrochemical reactor encased withinthe first housing, the first bioelectrochemical reactor including aplurality of anodes arranged between a cathode assembly, wherein thecathode assembly includes two gas-diffusion cathodes separated on twosides of the plurality of anodes and arranged longitudinally along aflow direction of the fluid through the first bioelectrochemicalreactor, the gas-diffusion cathodes able to allow oxygen to permeateinto the fluid within the first bioelectrochemical reactor. The secondmodular MFC device is fluidically coupled to the first modular MFCdevice and operable to bioelectrochemically process the treated waterproduced by the first modular MFC device to concurrently generateelectrical energy and digest organic matter in fluid of the treatedwater to produce a further treated water, and the second modular MFCdevice comprises: a second housing, and a second bioelectrochemicalreactor encased within the second housing, the second bioelectrochemicalreactor including a plurality of anodes arranged between a cathodeassembly, wherein the cathode assembly includes two gas-diffusioncathodes separated on two sides of the plurality of anodes and arrangedlongitudinally along a flow direction of the fluid through the secondbioelectrochemical reactor, the gas-diffusion cathodes able to allowoxygen to permeate into the fluid within the second bioelectrochemicalreactor.

Example C49 includes the device of any of the preceding or subsequentexamples among C48-C59, wherein one or both of the first housing and thesecond housing includes a rigid casing having a solid frame that allowsmodular components of one or both of the first bioelectrochemicalreactor and the second bioelectrochemical reactor, respectively, to bepositioned within and removed from an interior of the respectivehousing.

Example C50 includes the device of any of the preceding or subsequentexamples among C48-C59, wherein one or both of the first modular MFCdevice and the second modular MFC device has a first dimension in theflow direction that is larger than a second dimension perpendicular tothe first dimension.

Example C51 includes the device of any of the preceding or subsequentexamples among C48-C59, wherein one or both of the first housing and thesecond housing includes an input hole and output hole arranged onopposing sides of the respective housing along the flow direction.

Example C52 includes the device of any of the preceding or subsequentexamples among C48-C59, wherein one or both of the first housing and thesecond housing includes a first opening and a second opening on opposingsides of the respective housing perpendicular to the flow direction thatallow for air flow into one or both of the first bioelectrochemicalreactor and the second bioelectrochemical reactor, respectively, toprovide oxygen to the cathode assembly.

Example C53 includes the device of any of the preceding or subsequentexamples among C48-C59, wherein an anode of the plurality of anodes ofone or both of the first bioelectrochemical reactor and the secondbioelectrochemical reactor includes carbon fibers that protrude from aninterior cylinder.

Example C54 includes the device of any of the preceding or subsequentexamples among C48-C59, wherein one or both of the first modular MFCdevice and the second modular MFC device includes a double reactorconfiguration, the double reactor configuration including twobioelectrochemical reactors arranged in series along the flow directionand encased in the respective housing.

Example C55 includes the device of any of the preceding or subsequentexamples among C48-C59, wherein one or both of the first modular MFCdevice and the second modular MFC device includes a triple reactorconfiguration, the triple reactor configuration including threebioelectrochemical reactors arranged in series along the flow directionand encased in the housing.

Example C56 includes the device of any of the preceding or subsequentexamples among C48-C59, wherein each of the first modular MFC device andthe second modular MFC device bioelectrochemically processes thewastewater: oxidizing the organic matter in the wastewater fluid bybiologically-catalyzed oxidation using biological species within thebioelectrochemical reactor, which causes excretion of protons insolution that diffuse to a cathode of the cathode assembly and anextracellular transfer of electrons released during the oxidation to aconductive surface of an anode of the plurality of anodes, andelectrically transferring the electrons to the cathode through anelectrical circuit, which facilitates electrochemical reduction at thecathode by interaction of the protons, the electrons and oxygen toproduce hydrogen peroxide and/or new water.

Example C57 includes the device of any of the preceding or subsequentexamples among C48-C59, wherein each of the first modular MFC device andthe second modular MFC device is operable to bioelectrochemicallyprocess the wastewater under a flow rate of at least 100 mL/min and/orat least 120 L/day, and/or which can be operated for at least 10consecutive hours of operation.

Example C58 includes the device of any of the preceding or subsequentexamples among C48-C59, wherein each of the first modular MFC device andthe second modular MFC device is operable to generate the electricalenergy and produce the treated water with net-zero energy consumption.

Example C59 includes the device of any of the preceding examples amongC48-C58, wherein the wastewater includes sewage received from a domesticinfrastructure system, agricultural system, or industrial system.

Example C60 includes the system of any of examples C1-C20 configured toimplement the method of any of examples C21-C35.

Example C61 includes the device of any of examples C36-C48 configured toimplement the processing the pre-treated wastewater by thebioelectrochemical conversion process in accordance with any of theexamples C21-C35.

It is intended that the specification, together with the drawings, beconsidered exemplary only, where exemplary means an example. As usedherein, the singular forms “a”, “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. Additionally, the use of “or” is intended to include“and/or”, unless the context clearly indicates otherwise.

While this patent document contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this patent document in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described in this patent document should not be understoodas requiring such separation in all embodiments.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in this patent document.

What is claimed is:
 1. A method for energy generation and wastewatertreatment, comprising: pretreating a raw wastewater by removing at leastsome of solid particles from a wastewater fluid that produces apre-treated wastewater; processing the pre-treated wastewater by abioelectrochemical conversion process that generates electrical energyand concurrently cleans the pre-treated wastewater to produce treatedwater by digesting matter in the wastewater fluid; extracting thegenerated electrical energy for storage or transfer to an externalelectrical device; and outputting the treated water, wherein theprocessing the pre-treated wastewater by the bioelectrochemicalconversion process is implemented in a microbial fuel cell (MFC) devicecomprising a bioelectrochemical reactor encased within a housing, thebioelectrochemical reactor comprising a plurality of anodes spatiallyarranged between at least two cathodes, wherein an anode of theplurality of anodes includes carbon fibers that protrude from aninterior cylinder, and wherein the at least two cathodes are separatedon two sides of the plurality of anodes and arranged along a flowdirection of fluid through the bioelectrochemical reactor for the fluidto flow between the at least two cathodes, the at least two cathodesable to allow oxygen to permeate into the fluid within thebioelectrochemical reactor.
 2. The method of claim 1, wherein thebioelectrochemical conversion process includes: oxidizing organic matterin the wastewater fluid by biologically-catalyzed oxidation usingbiological species within the bioelectrochemical reactor, which causesexcretion of protons in solution that diffuse to a cathode of the atleast two cathodes in the bioelectrochemical reactor and anextracellular transfer of electrons released during the oxidation to aconductive surface of an anode of the plurality of anodes in thebioelectrochemical reactor, and electrically transferring the electronsto the cathode through an electrical circuit, which facilitateselectrochemical reduction at the cathode by interaction of the protons,the electrons and oxygen to produce hydrogen peroxide and/or new water.3. The method of claim 2, comprising: collecting the pre-treatedwastewater in one or more equalization tanks and modulating a steadyorganic load and flow of the pre-treated wastewater to thebioelectrochemical reactor.
 4. The method of claim 1, wherein thepretreating the raw wastewater includes at least one of: removing the atleast some of the solid particles having one or both of a large size of1 cm or greater and a large mass of 50 g or greater; or removingchemical species including one or more of sulfur species, grease or oilfrom the wastewater fluid.
 5. The method of claim 1, wherein the MFCdevice is included in an array of a plurality of modular MFC devicesarranged in a hydraulic series, wherein a first modular MFC devicereceives the pre-treated wastewater and bioelectrochemically processesthe pre-treated wastewater to output a first treated water that isreceived at a second modular MFC device of the array, whichbioelectrochemically processes the first treated water to output asecond treated water outputted from the array or received at a nextmodular MFC device of the array.
 6. The method of claim 2, wherein thebiological species include at least one of a bacteria, a yeast, or aconsortium of bacteria and yeast.
 7. The method of claim 1, wherein theoutputting the treated water includes one or both of storing the treatedwater in a tank and routing the treated water to an external fluidicdevice.
 8. The method of claim 5, wherein pre-treated wastewater isgravity-fed to the modular MFC device.
 9. The method of claim 1, theprocessing the pre-treated wastewater to generate the electrical energyand concurrently produce the treated water occurs under a flow rate ofat least 100 mL/min and/or at least 120 L/day.
 10. The method of claim9, wherein the processing the pre-treated wastewater to generate theelectrical energy and concurrently produce the treated water occursunder the flow rate of at least 100 mL/min and/or at least 120 L/day forat least 4 consecutive hours of operation of the method.
 11. The methodof claim 1, wherein the processing the pre-treated wastewater by thebioelectrochemical conversion process generates the electrical energyand produces the treated water with net-zero energy consumption for atleast a portion of an implementation of the method.
 12. The method ofclaim 1, wherein the raw wastewater includes sewage received from adomestic infrastructure system, agricultural system, or industrialsystem.
 13. The method of claim 1, wherein the at least two cathodesinclude two gas-diffusion cathodes operable to allow oxygen to permeateinto the fluid within the bioelectrochemical reactor.
 14. A method forenergy generation and wastewater treatment, comprising: pretreating araw wastewater by removing at least some of solid particles from awastewater fluid that produces a pre-treated wastewater; processing thepre-treated wastewater by a bioelectrochemical conversion process thatgenerates electrical energy and concurrently cleans the pre-treatedwastewater to produce treated water by digesting matter in thewastewater fluid; extracting the generated electrical energy for storageor transfer to an external electrical device; and outputting the treatedwater, wherein the processing the pre-treated wastewater by thebioelectrochemical conversion process is implemented in a microbial fuelcell (MFC) device comprising a bioelectrochemical reactor encased withina housing, the bioelectrochemical reactor comprising a plurality ofanodes spatially arranged between at least two cathodes, wherein ananode of the plurality of anodes includes carbon fibers that protrudefrom an interior cylinder, and wherein the at least two cathodes includetwo gas-diffusion cathodes separated on two sides of the plurality ofanodes and arranged longitudinally along a flow direction of fluidthrough the bioelectrochemical reactor, the gas-diffusion cathodes ableto allow oxygen to permeate into the fluid within the bioelectrochemicalreactor, and wherein the MFC device is included in an array of aplurality of modular MFC devices arranged in a hydraulic series, whereina first modular MFC device receives the pre-treated wastewater andbioelectrochemically processes the pre-treated wastewater to output afirst treated water that is received at a second modular MFC device ofthe array, which bioelectrochemically processes the first treated waterto output a second treated water outputted from the array or received ata next modular MFC device of the array.
 15. The method of claim 14,wherein the processing the pre-treated wastewater by thebioelectrochemical conversion process generates the electrical energyand produces the treated water with net-zero energy consumption for atleast a portion of an implementation of the method.
 16. The method ofclaim 14, wherein the raw wastewater includes sewage received from adomestic infrastructure system, agricultural system, or industrialsystem.
 17. The method of claim 14, comprising: collecting thepre-treated wastewater in one or more equalization tanks and modulatinga steady organic load and flow of the pre-treated wastewater to thebioelectrochemical reactor.
 18. The method of claim 14, wherein thepretreating the raw wastewater includes at least one of: removing the atleast some of the solid particles having one or both of a large size of1 cm or greater and a large mass of 50 g or greater; or removingchemical species including one or more of sulfur species, grease or oilfrom the wastewater fluid.
 19. The method of claim 14, wherein theoutputting the treated water includes one or both of storing the treatedwater in a tank and routing the treated water to an external fluidicdevice.
 20. The method of claim 14, wherein pre-treated wastewater isgravity-fed to at least some of the modular MFC devices.